sql tutorial

SQL - Structured Query Language

SQL stands for Structured Query Language and is used to pull information from databases.SQL offers many features making it a powerfully diverse language that also offers a secure way to work with databases.

SQL - Fundamentals

SQL alone can input, modify, and drop data from databases. In this tutorial we use command line examples to show you the basics of what we are able to accomplish. With the use of web languages such as HTML and PHP, SQL becomes an even greater tool for building dynamic web pages. To get started you will need a working database program such as Oracle, DB2, SQL Server, or Mysql.

For more information and installation help on either of the above database programs, we suggest you go straight to the developer homepages at:

OracleSQL ServerMySQL

SQL - World Wide Web

SQL has become a popular among web designers due to its flexibility and simplicity. With some basic knowledge of HTML, PHP, and a database program such as MySQL, a web designer is capable of creating some of the most complex of web sites. Most of the concepts demonstrated here in the tutorial are also great for web programming.

SQL - What's a Database?

A database is nothing more than an empty shell, like a vacant warehouse. It offers no real functionality what so ever, other than holding a name. Tables are the next tier of our tree offering a wide scope of functionality. If you follow our warehouse example, a SQL table would be the physical shelving inside our vacant warehouse. Each SQL table is capable of housing 1024 columns(shelves). Depending on the situation, your goods may require reorganization, reshelving, or removal. SQL tables can be manipulated in this same way or in any fashion the situation calls for.

SQL - Four Principles of Database Design

When designing and implementing a database, keep in mind these four guidelines.

AtomicityYour coded statements flow without the constant need to update or "fix" your data.ConsistencyYour statements are either executed 100% or fail 100%, do not implement code that partially works.IsolationKeep data files and logs away from public eyes, and limit the number of users with administration access to your database.DurabilityMaintain reliable servers with plenty of storage space and back-up systems that save transactions immediately.

http://www.mysql.com/

http://www.microsoft.com/sql/default.mspx

http://www.oracle.com/index.html

A well thought out database will continue to serve and meet your needs for ages. It is important to plan ahead and really put some thought into what you intend to record in your databases. Keep in mind that tables and databases should maintain some relationship. In many instances it is far more desirable to have several small related tables and databases than one giant one. "Never place your eggs all in one basket."

The Database Model

Dr. Edgar F. Codd was the founding father of what is known as the relational model of databases. In 1970, he published a groundbreaking article "A Relational Model of Data for Large Shared Data Banks." Included within the article were 12 rules of relational databases. These rules are as follows (paraphrased).

1. Information RuleAll data in the database should be represented as values in tables.

2. Guaranteed AccessEvery piece of data should be accesible by using the table name, primary key, and a column name.

3. Treatment of NULL ValuesNull values must be treated as incomplete data pieces. Nulls are not to be confused with zeros.

4. Active-Online Relational CatalogA database is represented at a logical level of tables.

5. SublanguageHaving one supported language with a well-defined syntax.

6. Updating ViewsAll views should be updated through the system.

7. Set-level Insertion, Update, and DeletionSystem must support set-time insert, update, and deletion operations.

8. Data Independence (Physical)Alterations to the way data is stored must not alter the application itself.

9. Data Independence (Logical)Altering tables, columns, and/or rows must not alter the application itself.

10. Integrity IndependenceThe language of the database must define integrity rules.

11. Distribution IndependenceDistributing the database to numerous locations should be anonymous and existing applications should be unaffected.

12. NonsubversionIf the system uses a low level interface to record data, then there must be a higher level interface used when administrating.

The largest of corporations follow these rules of cataloging information to this very day.

Database Driven Web Sites

Today we experience the power of databases throughout the web. Many sites are completely dynamic, meaning the content that is being display is held inside of a table with columns, and columns of information to display. This is quickly becoming the ideal way to host websites. It allows for dynamic content on the fly, user interaction, and webmasters can store information about returning users making way for the site to recognize returning users. Any chance at building rapport with your audience is a great opportunity indeed.

SQL - Platforms

A SQL platform acts as the stage for building and developing your databases. Several different platforms exist including:

← IBM's DB2 ← MySQL ← PostgreSQL ← Oracle ← Microsoft's SQL Server

SQL - MySQL and PostgreSQL

MySQL and PostgreSQL are open source database programs rich in functionality and flexibility. They are often the choice of web developers and small businesses simply because they get the job done for a very reasonable price. Also they will go anywhere and can operate on nearly every operating system available.

SQL - SQL Server

Microsoft's SQL Server is steadily on the rise in the commercial world gaining popularity slowly. This platform has a GUI "Windows" type interface and is also rich with functionality. A free trial version can be downloaded at the Microsoft web site, however it is only available to Windows users.

SQL - DB2 and Oracle

By far the selection of choice for large corporations is either Oracle or DB2. Companies that have large ties to IBM stick to their DB2 software whereas others have made the switch to Oracle. These systems run on personal computers as well as large corporate mainframes.

SQL - Queries

Queries are the backbone of SQL. Query is a loose term that refers to a widely available set of SQL commands called clauses. Each clause (command) performs some sort of function against the database. For instance, the create clause creates tables and databases and the select clause selects rows that have been inserted into your tables. We will dive deeper in detail as this tutorial continues but for now let's take a look at some query structure.

Query construction begins with one of the following clauses:

← Add ← Drop ← Create ← Insert ← Select ← Update ← Replace ← Delete

Queries are loosely typed into your SQL prompt. Spacing and line breaks are not very important as we will discuss further in our SQL Syntax lesson. We now know that a query begins with a clause, what comes next depends on the clause we select and we will be covering all the clauses as the tutorial progresses. For now, let's take a look at some syntax.

SQL - Query Syntax

The syntax of a query is loose, meaning you are free to place line breaks where you please without injuring the code. Few instances require parentheses, including the insert statement listed below. Parentheses will also be covered during our Functions lesson. Be sure to end all query statements with a semicolon (;).

SQL Code:SELECT * FROM table_name;

The above code selects every row and every column from a hypothetical table (table_one) and prints it to our prompt. Here's a look at a few more queries that should become second nature to you as the tutorial coninues.

SQL Code:INSERT INTO table_name (column_one,column_two) VALUES(value_one,value_two);

SQL Code:UPDATE table_name SET column_one = value_one, column_two = value_two;

Queries are how you communicate to your database program. Nearly everything typed at a SQL command prompt is a query.

SQL - What are Subqueries: Advanced Queries

Subqueries can be performed inside of existing queries. This expands the capabilities of SQL in a number of ways providing a 3rd dimension to the language you might say. Again, we will discuss subqueries in more detail in a later lesson. Feel free to familiarize yourself with what a subquery may look like using the example below.

SQL Code:SELECT * FROM table_one WHERE unique_column = (SELECT unique_column FROM table_two WHERE id_column = 1)

Above is a look at where you might cross subqueries. The logic behind the entire query is fairly confusing at this point, try and stay with us and focus on the syntax of the query and subquery.

SQL - Syntax

SQL follows a general syntax, there are not many quotations or other symbols to throw into your statements. Generally we follow a Do what To what syntax, meaning first we decide what we want to do, then we decide what we want to do it to, and finally we end the whole thing with a semicolon (;).

A statement begins with a clause. Clauses are commands in the SQL world and the backbone of any script. The first clause of a statement gives a general idea of what type of action a script is taking. A few basic clauses are SELECT, INSERT, or CREATE. We will look at each of these clauses a little more in depth on the next few pages but it may be obvious to you already what each of those clauses does. SQL statements end with a semicolon as most with most programming languages. A basic statement might look like this:


Above we have a SELECT clause asking for all columns and values (*) from our database table. As shown above, a good habit is to capitalize your clauses. Later on when we have larger statements and subqueries it will make life much easier to go back and debug your code.

Formating your statements in a similar fashion will also aid your debugging efforts. The common formatting technique is to begin each line with a clause or to break up and list columns or tables as needed. More on this in a moment.


The advantage of this isn't apparent with this example. Each are fairly easy to read. However the example below shows an example where this format shines.

SQL Code:SELECT column_one, column_twoFROM table_nameWHERE (column_one,column_two,column_three,)= (SELECT column_one,column_twoFROM table_twoWHERE table_one.id = 'table_two.id');

As you can see, when subqueries are thrown into the mix things become a little more complicated. A one line statement will not fit across your screen. Both statements are neither right nor wrong, each are easier to follow. Parentheses generally depict order of operations but it is not an exact science. Quotations are not found until the predicate of the statement.

SQL - Data Types

SQL recognizes 4 general types of data. As the database designer you will be selecting which type of data that can be placed in each table column. Before we look at each type of table column we will elaborate on specific data types and how they are handled in SQL.

← Character Strings - ('Words or numbers') ← Numbers - (3, 3.423, -17) ← Booleans - (True / False) ← Nulls - (empty fields)

SQL - NULL Values

A null value may be the most foreign to new programmers. Stating that a value has a null value indicates that nothing exists in that table field. When the table is created you may either allow a table to have a null value or may disallow null values for each table column.

SQL Code:CREATE TABLE weekly_payroll(employee_id VARCHAR(10) PRIMARY KEY,total_hours INT NULL,hourly_rate MONEY NOT NULL,);

SQL - Numeric Data

Dates, time stamps, integers, and money are all numeric data types. The advantage of working with numbers is that SQL has built in functions such as the AVG() or SUM() functions that will return the average or the sum of a numeric column.

Numbers:rate_of_pay

27

26.66

28.40

SQL - Boolean Data

Boolean values are either yes/no (true/false) types of data. Others use a 1/0 (1 for yes 0 for no) approach. Either something is or something is not.

Boolean Values:admin

1

1

0

SQL - Character Strings

Character strings are sentences, symbols, or a combination of both. Math functions can not be performed with character strings.

Character Strings:employee_id

TS_0036

TS_0078

CL_1099

SQL - Commands

SQL commands can be categorized into three distinct groups. Each type of command plays an essential role while working with your database. One analogy might be to think of each SQL Command as a possible tool in your tool shed. Certain duties require specific tools and the more tools you have in your shed, the greater the chances that you will have the exact tool you need for the appropriate job.

SQL - What is a Clause

Clauses were mentioned briefly in the SQL Queries lesson. To briefly recap, they are the commands issued during a query. SELECT, INSERT, ADD, DROP, CREATE, etc are all clauses that begin each SQL Query and execute some sort of action upon your database.

SQL - What is a Function

There are a number of functions built into SQL that can add column values for you, average column values, or even change lowercase to uppercase strings. These functions are used directly inside of your queries and are excellent tools for you to use.

SQL Code:Count() function

SELECT COUNT(*) FROM table_one;

The query above will return a numeric value representing the number of rows that have been inserted into your database. We will be covering this function as well as many others as this tutorial continues.

SQL - What is an Operator

Operators are a means by which SQL can manipulate numbers and strings or test for equality. They come in four flavors including: Arithmatic, Range, Equality, and Logical. As your skills with SQL grow, you may want the language to start performing some basic arithmatic for you or perhaps you wish to select a range of rows with a numeric column value larger than 5. This becomes possible with operators.

SQL Code:SELECT * FROM table_one WHERE column_one > 5;

http://www.tizag.com/sqlTutorial/sqlqueries.php

SQL - Operators

Operators are used in expressions or conditional statements. they show equality, inequality, or a combination of both. Mathematical operators are found in every computer language and may be familiar to you. SQL operators follow the same rules you may have learned in a math class or know from previous programming experience.

Operators come in three flavors, mathematical, logical, or range operations. Mathematical operators add, subtract, multiply, divide, and compare equality of numbers and strings. There are two logical operators, AND / OR. Range operators include the infamous < and > symbols used to compare equality. Take note of the following tables for future reference.

SQL Arithmetic Operators:Operator Example Result Definition

+ 7 + 7 = 14 Addition

- 7 - 7 = 0 Subtraction

* 7 * 7 = 49 Multiplication

/ 7 / 7 = 1 Division

% 7 % 7 = 0 Modulus

Modulus may be the only unfamiliar term on the chart. this term describes the result when one number is divided by another number resulting in a remainder. For example 4 % 3 would result with a value of 1 since 1 is left over after 4 is divided by 3.

SQL Range Operators:Operator Example Defined Result

< 7 < 4 7 less than 4? False

> 7 > 4 greater than 4? True

<= 7 <= 11 Is 7 less than or equal to 11? True

>= 7 >= 11 Is 7 greater than or equal to 11? False

SQL Equality Operators:Operator Example Defined Result

= 5 = 5 Is 5 equal to 5? True

<> 7 <> 2 Is 7 not equal to 2? True

SQL Logical Operators:Operator Defined Example

AND Associates two values using AND if (($x AND $y) == 5)...

OR Associates two values using OR if (($x OR $y) == 5)...

SQL - Expressions

In the programming world an expression is a special statement that returns a value. SQL is no exception to this standard rule.

SQL - Expression Types

Expressions in SQL generally fall into one of four categories including: Boolean, Numeric, Character, and/or Date Expressions.

SQL Code:SELECT column_one FROM table_name;

The simplest form of an expression appears as column_one of our table. Select is our clause telling our database what we want to do, and column_one acts as the defined expression returning each row of that particular column.

Expressions after the where clause might appear more familiar to programmers.

SQL Code:SELECT * FROM table_name WHERE column_one = 'some value';

The latter example returns rows of the specified column containing 'some value'. Using expressions like the one above gives you precise control over what results will be returned. More information on the where clause is available at SQL Where.

SQL - Boolean Expressions

A boolean expression in any programming language returns a true/false result. Returning to the previous example containing the where clause.

SQL Code:SELECT * FROM table_name WHERE column_one = 'some value';

The logic behind this query is that each row is being tested for 'some value' to appear in our column_one. Each time a match is found (testing true), that row is selected and returned for our viewing pleasure.

SQL - Numeric Expression

A numeric expression simply returns a numeric value. There are some built in functions that we will be examining in greater detail later on. Using one of the following functions is perhaps the easiest way to demonstrate the return of a number:

← AVG() -- Returns the average of a stated column. ← COUNT() -- Returns a count of the number of rows of a given column. ← SUM() -- Returns the sum of a given column.

SQL Code:SELECT COUNT(*) FROM table_name;

http://www.tizag.com/sqlTutorial/sqlwhere.php

Our expression above returns a numeric value representing the number of rows that have been inserted into your table thus far. Please be aware that the AVG(), COUNT(), and SUM() only return results for integer table columns. Using one of these functions with a varchar column will result in an error message.

SQL - Character Expressions

Character expressions are used to test for values of a string. Often these expressions will appear in a where clause as follows.

SQL Code:SELECT * FROM table_name WHEREcolumn_one LIKE '%string';

Here we have used the percent(%) symbol to signify the start of our string. SQL tests our expression against column_one and returns all the rows and columns where column_one contains our string.

This might come across easier if we use a live example. Say we have created a table with employee information. In this table we have set up a column a last_name column. The query above will come in handy if we were wanting to pull all the employees with a last_name that begins with a "T". Now if we plug in our hypothetical situation into our code, we should have something like the following.

SQL Code:SELECT * FROM employees WHERElast_name LIKE '%T';

Keep in mind that SQL is case sensitive, using a lowercase t would not yield results for a last_name that has been capitalized.

SQL - Date Expressions

Date expressions come in three flavors. These expressions are very straight forward, simply type in any of those listed below and SQL will return exactly what you have requested.

← Current_Date -- Returns the current date. ← Current_Time -- Returns the current time. ← Current_Timestamp -- Returns the current timestamp.

These expressions can also be placed into your tables as column values for any given row with an insert statement. We will be looking more indepth at the insert clause, however here is a glimpse of what is to come.

SQL Code:INSERT INTO table_name(column_one,column_two,)Values(Current_Date,Current_Timestamp);

This statement inserts a new row into our imaginary table with the current date value for column one and the current timestamp value for columne_two.

SQL - Create

A database is nothing more than an empty shell, like a vacant warehouse. It offers no real functionality what so ever, other than holding a name. Tables are the next tier of our tree offering a wide scope of functionality. If you follow our warehouse example, a SQL table would be the physical shelving inside our vacant warehouse. Depending on the situation, your goods may require reorganization, reshelving, or removal. SQL tables can be manipulated in this same way or in any fashion the situation calls for.

SQL - Create Database

We first need to build our warehouse, or create our database. The create clause is straight forward. Here we have a one line script using the create clause to create a database we named business. The exact number of databases a SQL program can handles is entirely up to the manufacturer, visit your sites manufacturer if you would like to have specifics.

SQL Code:CREATE DATABASE business;

We will be using this database for the remainder of the tutorial.

SQL - Create Table

Our emtpy shell of a database will do nothing standing alone, next we create our tables to store and organize our data. We use the same create clause and follow the same syntax above. Tables, however, are fairly complex. In our script we must also include parameters for each table column as well as name each one. Naming your table columns can be as simple or complicated as desired. SQL programs are case sensitive, keep this in mind as you will be calling on our table columns by name a great deal as you enhance your SQL knowledge. Also, most programs do not support spaces in column names, you must use the underscore (_).

Column types specify what type of data can be placed inside a table column ranging from numbers, paragraphs, or brief strings. For example, setting a column type to an int value means your database will ONLY accept an interger value for this column type.

Column Types:Column Type Description Syntax

int Accepts integer values only tinyint, int

varchar Accepts any symbol, character, or number varchar(char limit value)

text/blob lots of text including line breaks text, blob

Int, varchar, and text are the 3 most common types of columns. Text and int columns have 3 flavors tiny, medium, and large. Every SQL program has its unique sizes, but for this tutorial we will be using medium sized column fields for each exercise. Later on as your database exapands it becomes extremely important to not overdue the size of your column fields. Using the correct size field will dramatically increase performance including query speeds.

Now create the table.

SQL Code:CREATE TABLE employees(id INT(3) NOT NULL AUTO_INCREMENT,Lastname VARCHAR(50),Firstname VARCHAR(25),Title VARCHAR(10) DEFAULT 'crew' NULL);

Above is our table, the first column simply numbers each row that will be added to the table up to a maximum of 3 digits wide (999) automatically(auto_increment). Our second line is a varchar meaning it will hold numbers, digits, or symbols, which is perfect for short names of people. The last column has a specified default value of "crew" so that if we add a new crew member we can simply place a default value.

We do not always have to specify a default value or state weather each column may have a NULL value, By default, most table columns will allow null values to be recorded but certain table columns will not allow a null value to be inserted. We recommend reading up on each column type available to you from your database manufacturer.

SQL - Primary Key

A primary key is a property given to a table column that distinguishes that record apart from each. For each record in the table the primary key acts like a driver's license number, only one number exists for each person. The same principle applys here. Any table can only be given one auto increment field and as such it is forced to be the primary key of the table. Therefore in our following examples we use the 'id' field as our primary key.

The alter clause is used to add or drop primary keys and indexes. To change a primary key you must drop the first one, then add your desired primary as shown below. More about the alter clause later.

SQL Code:ALTER TABLE 'employees' DROP PRIMARY KEY,ADD PRIMARY KEY ('id');

SQL - Indexes

SQL automatically creates some indexes based on column types and attributes. An index can also be given to a table column to optimize speeds. When a query is executed searching for a specific column value, SQL will start at the top of the table and search each and every record until it finds matches. This becomes a performance issue when a table holds a vast amount of records. By adding an index to columns, SQL will no longer search the entire table, it will pinpoint your index columns and search those first. The downside to indexing is that it enlarges the disk space consumed by a table on your webserver. Use indexes when you notice a drop in your query speeds.

SQL Code:ALTER TABLE 'employees' ADD INDEX ('id');orALTER TABLE `employees` DROP INDEX `Lastname`;

SQL - Insert

The insert clause has one function; to insert data into a table. Insert populates each table column with a value. Rows are inserted one right after another into the corresponding column.

SQL Code:INSERT INTO employees (Lastname,Firstname,Title) VALUES(Johnson,David,crew);

Above we have a single line example of the insert syntax. Since our 'id' column is set up to auto increment, we can omit this field from our script. Our SQL program automatically begins counting starting with one, when the auto increment attribute is added to an integer field.

Display:id Lastname Firstname Title

1 Johnson David crew

SQL - Insert defaults and nulls

We mentioned setting up default or null values for table columns. Simply placing the word default or null, in place of a value is the solution.

SQL Code:INSERT INTO employees (Lastname,Firstname,Title) VALUES('Hively','Jessica',DEFAULT);orINSERT INTO employees (Lastname,Firstname,Title) VALUES('Hively','Jessica',NULL);

SQL - Inserting multilpe values

Here's an example of how to insert more than one record at a time. Many web developers will use the single example above along with HTML forms to continually insert and update their SQL tables.

SQL Code:INSERT INTO employees VALUES(DEFAULT,'Hicks','Freddy','crew'),(DEFAULT,'Harris','Joel','crew'),(DEFAULT,'Davis','Julie','manager');

We use a default value for the id field so that it will continue to auto increment for each new record. Using this method you must have some value for each table column.

SQL - Insert into multiple tables

This concept isn't widely supported by open source database programs, however they do offer alternative methods to achieve the same goal. The idea is to insert similar record values into 2 or more tables with one statement. Using the example from above, we want to place Julie's information into our manager table as well as the general employee table.

SQL Code:INSERT ALL INTO employees (Lastname,Firstname,Title) VALUES('Davis','Julie','manager')INTO manager (training,salary)VALUES ('yadayada','22500');

SQL - Select

In SQL, queries begin with the select clause. A typical query statement needs only two parts, we select what from where. The what will represent columns of your table you wish to select and the where represents the table name of your data table.

To avoid typing a list of every single column in your table, you can use the astric (*) to select all table columns from a table. However, query speeds are very important and always selecting every table column can dramatically decrease your SQL performance. As good practice, it is best to only select the table columns that you wish to use or display.

SQL Code:SELECT * FROM employees;

SQL Table:ID Lastname Firstname Title


2 Hively Jessica crew

9 Hicks Freddy crew

10 Harris Joel crew

11 Davis Julie manager

Our result should print every table column and every bit of data stored in each row thus far.

SQL Code:SELECT Lastname,Firstname FROM employees;

SQL Table:Lastname Firstname

Johnson David

Hively Jessica

Hicks Freddy

Harris Joel

Davis Julie

SQL - Select Multiple Tables

Multiple columns from different tables can also be selected. The syntax is relatively the same, the difference being when you choose that table column you wish to select, you must now name the table of the desired table column.

The period is necessary to differentiate between the table name and the column name, other than that all the same query rules apply.

SQL Code:SELECT employees.Fisrtname, employees.Lastname, crew.experience FROM employees, crew;

SQL - Select Functions

We havn't mentioned anything about SQL Functions yet and we will eventually cover them in greater detail, for the moment we would like to introduce this concept of using a selection query with a function. For instance we can query our database program for the current_timestamp or retrieve the number of rows in our table using the COUNT() function.

SQL Code:SELECT CURRENT_TIMESTAMP;

This will return a server timestamp representing the date and time of when the query was executed, more on this toward the end of the tutorial.

SQL Code:SELECT COUNT(Lastname) FROM employees;

We will dive deeper into detail as the tutorial progresses, but be aware of this concept of selecting date by means of calling a function as it will prove more useful later on.

SQL - Order By

The order by statement allows for table column assortment. It allows for ascending or descending lists of your table column values permitting SQL to reorder your table rows for the purpose of viewing.

SQL Code:SELECT * FROM employees ORDER BY Lastname;



10 Harris Joel crew

9 Hicks Freddy crew



The above example resorts our rows of data alphabetically by lastname. Here is another example ordering by two different columns First we alphabatize our job titles and again we order by lastname.

SQL Code:SELECT * FROM employees ORDER BY Title,Lastname;


10 Harris Joel crew

9 Hicks Freddy crew




Hint: It is possible to order by a column you do not wish to display such as an ID field. Also notice at the bottom of each query display will be the number of rows affected by your query. This number will also be displayed anytime an insert, select, update, or delete statement is properly executed by your SQL program.

SQL - Distinct Usage

Distinct is used to retrieve rows of your database that have unique values for a given column. For example say we have a table of employees and in this table of employees we have several job titles from janitors to CEOs. We would like to know just how many distinct job titles we have.

SQL Code:SELECT DISTINCT job_titles FROM employees;

Our statement will return the number of rows with a distinct value for our column (job_titles).

SQL - Distinct Multiple Columns

Using the distinct function with more than one column yields some substantial results. SQL will return the rows with distinct or unique combinations of those columns. Assume this same employee table has another column including the salary of each employee. With the use of the distinct function we can pull a list of unique job titles - salaries.

SQL Code:SELECT DISTINCT job_titles, salary FROM employees;

SQL has returned all the rows with unique combinations of job titles and salaries. Any duplicate combinations of job titles and salaries will be ignored. For example if we had two CEOs in the table making the same salary each year, only one row would be returned but if we had two CEOs with different salaries, both rows would be returned.

SQL - Where

The where clause sets a conditional statement for your queries. Your server will query the database searching for rows that meet your specific where condition. A typical query will have the following syntax.

A conditional statement has 2 parts, the left side is your table column and the right side is the condition to be met. Example:WHERE table_column(s) = condition.

SQL Code:SELECT * FROM employees WHERE Title = 'crew' ORDER BY Lastname;


10 Harris Joel crew

9 Hicks Freddy crew



All the known typical operators can be used in the conditional statements, for a complete list backtrack to SQL Operators

SQL - Where multiple conditionals

SQL also supports multiple conditional statements within one script by using the AND or OR operators. Continuing the example from above we can select only those employees that worked 36 or more hours for the week, perhaps meaning they are full time employees.

SQL Code:SELECT employees.Lastname,employees.Firstname,employees.Title,payroll.Hours

http://www.tizag.com/sqlTutorial/sqloperators.php

FROM employees,payrollWHERE employees.Lastname = payroll.LastnameAND payroll.Hours => '36'ORDER BY employees.Lastname;

As you can see, the where clause is a very powerful tool used to quickly pull rows from one table or many.

SQL - In

In is a special kind of operator for use in your where clauses. Recall that in a where expression only one value is allowed to be sent through the query. With the in operator one can send multiple values in the where clause.

SQL Code:SELECT * FROM table_nameWHERE some_column IN('value1','value2','value3');

The rows returned will have one of the values listed (value1, value2, or value3) in brackets as the value for the specified column (column_one).

SQL - In Subqueries

To better understand the power of this function, let's use an example involving a subquery. Say we have a database with the following tables.

Employees: user_id last first

0045 Davis Julie

0067 Smith John

0098 Hodgensen Bruce

Logbook: user_id timestamp

0045 000000002345

0045 000000045666

0098 000000076444

Now above we have an employees table giving us a user_id along with a first and last name of employees we have working in the office. The second table might be a log type table that keeps track of who accessed the database and at what time. This a security check. A standard operating procedure if many people will be accessing the database.

What we want to know is the last name and first name of those that accessed the database. To do this we may use a subquery like in the example below.

SQL Code:SELECT first,last FROM employees WHERE user_id IN(Select user_id FROM logbook);

The subquery highlighted in red selects all the values of user_id in the logbook and returns those to the previous in statement. The result is a complete listing of the names of the employees that have accessed the database and have been recorded inside the logbook table. The good news is that this feat was accomplished without having to know the user_ids of every employee in the office. This definatly adds some depth to your SQL knowledge thus far.

SQL - Between

Between is a special operator appearing in a where statement. It allows for the selection of a range of values between one value and another.

SQL Code:SELECT * FROM table_name WHERE column_oneBETWEEN value1 AND value2;

Be aware that the values you specify will also return results. For example if we were looking for values between 5 and 10, all rows would be retrieved where our column value is 5,6,7,8,9, or 10.

SQL - In Between (Subqueries)

This next example requires that you are familiar with the previous lesson, SQL In. You may recall that the In operator allows the where clause to return more than one single value for a column. Below are two tables. The employee table only stores personal information about employees while the logbook records when and who accessed our database.

employees user_id last first

0045 Davis Julie

0048 Thomas David

0067 Smith John

0098 Hodgensen Bruce

logbook user_id timestamp

0045 000000002345

0045 000000045666

0048 000000055767

0098 000000076444

Say we were thinking ahead when issuing user_ids and coordinated each user_id number with a department. For example our data security department employees will be numbered 004X, where X represents any number 0-9 (So data security employees are numbered 0040-0049). Since we konw this information it will prove useful if we want to track down database users by department.

SQL Code:SELECT * FROM employees WHERE user_id IN(SELECT user_id FROM logbook WHERE user_id BETWEEN 0040 AND 0049);

SQL will now retrieve exactly what we need. A listing of all the data security employees (user_id 004X) that have accessed the database thus far.

SQL - Alter Table

The alter clause changes a table by adding, removing, or modifying an existing table column.

The following example adds a new column to our table.

SQL Code:ALTER TABLE employeesADD dob varchar(10);

SQL Table:id Lastname Firstname Title dob


Now we will remove the same column from the table.

SQL Code:ALTER TABLE employeesDROP dob;

SQL Table:id Lastname Firstname Title


SQL - Alter Columns

Alter is used again when changing attributes of the table cloumns. For instance if we wanted to change a table column name or change a column from a varchar to an interger type.

SQL Code:ALTER TABLE 'employees' CHANGE 'ID' 'id' TINYINT ( 3 ) NOT NULL AUTO_INCREMENT;

Above we changed 'ID' to 'id' because SQL is case sensative and this will remove a potential bug issue. After we have selected a new name, we list the new attributes to be given to the altered column.

SQL - Update

The update clause updates column values of a table. Update requires a conditional statement to select the row to be updated.

SQL Code:UPDATE employeesSET Lastname = 'Fletcher', Firstname = 'Ron'WHERE id = '11';


11 Fletcher Ron manager

Here we changed record 11, our manager, to a new manager.

SQL - Updating Multiple Rows

With the use of expressions and operators, update allows for the manipulation of several rows at once.

SQL Code:UPDATE employeesSET Title = UPPER(Title);


1 Johnson David CREW

2 Hively Jessica CREW

9 Hicks Freddy CREW

10 Harris Joel CREW

11 Davis Julie MANAGER

Using the UPPER expression we changed our Title field to all capital letters. Update also supports the use of subqueries.

SQL - Delete

Entire rows can be deleted from a table using the delete clause.

SQL Code:DELETE FROM employeesWHERE id='4';

We could use the above code to delete an employee using their unique employee number that has been assigned by the auto_increment field.

SQL - Deleting Multiple Rows

Delete is not limited to one row at a time. We may use all of our known SQL operators as well as subqueries to delete any or all rows that apply to our conditional.

SQL Code:DELETE FROM employeesWHERE id <= '999';

The above example would delete every row from our table since we had previously limited our "id" field when we set up the table. Predicates, expressions, subqueries, and operators can be used to delete any rows you would like to remove from your table.

SQL - Truncate

We can clear an entire table using a truncate statement. Truncate quickly clears all rows of a table without deleting the table itself. If you are following along, we don't recommend that you run this script because you will essentially undo all of your inserts and have only empty table field to show for it, but it is a handy statement to know and use when the time is right.

SQL Code:TRUNCATE TABLE employees;

SQL - Predicates

Predicates follow the where clause. Predicates allow the searching through database records to recover specific strings and ranges or characters. Rows will be returned if they match the predicate condition.

SQL - Like; Not Like

Like is a means of quering data in your database by keyword or keyletter means. After a where clause, use LIKE to match a character or a string. SQL retrieves all rows containing all or part of the string placed within the parameters. Generally, a percent sign (%) is used to define the start and ending or your string. Escape characters can be used for special situations.

SQL Code:SELECT * FROM employees WHERE Lastname LIKE '%H%';


10 Harris Joel crew

9 Hicks Freddy crew


Our example will retrieve any and all rows with a capital letter H in the lastname field. Case sensitivity is important in this situation.

On a contrary note, use the Not Like predicate to find all rows that do not match the string.

SQL Code:SELECT * FROM employees WHERE Lastname NOT LIKE '%H%';




SQL - Predicates Escaping Characters

Say you want to find a percent character(%) in your database, or all rows associated with one. SQL Server and Oracle, require an additional statement over MySQL. With Oracle and SQL Server, you must specify the character used to escape. Here's code example.

SQL Code:

Oracle/SQL Server

SELECT * FROM employees WHERE Lastname LIKE '%\%%' ESCAPE '\';

MySQL

SELECT * FROM employees WHERE Lastname LIKE '%\%%'

MySQL has a built in default escape character the backslash (\). MySQL also supports the escape predicate allowing you to change your escape character exactly as it is done in the other programs.

SQL - Between

Between is a predicate to call a range of numeric values such as 1-20 or the like. Its syntax follows the same as above.

SQL Code:SELECT * FROM employees WHERE id BETWEEN 1 AND 4;




Between is essentially replacing your range operators. Always start your ranges with the lowest number first.

SQL - Limit

The limit predicate allows you to limit the number of rows selected.

SQL Code:SELECT * FROM employees LIMIT 2;




SQL - Inner Join

The join clause combines columns of one table to that of another to create a single table. Join matches up a column with one table to a column in another table. A join query does not alter either table, but temporarily combines data from each table to be viewed as a single table. There are three types of join statements, inner, left, and right.

We will be using our employees table from previous examples, and a new table to track sales of each employee called invoices.

Our invoices table is set up with 3 fields, EmployeeID, Sale, and Price. If we were a business owner we now have a means to track what was sold, and by whom and we can bring this information together using an inner join clause.

Inner Join: An inner join returns all rows that result in a match such as the example above.

SQL Code:SELECT employees.Lastname, employees.Firstname, invoices.Sale, invoices.PriceFROM employeesINNER JOIN invoicesON employees.id = invoices.EmployeeID

SQL Table:Lastname Firstname Sale Price

Johnson David HOT DOG 1.99

Hively Jessica LG SFT DRK 1.49

Davis Julie CD SLD 3.99


We haven't changed or updated any information in either of our tables but we were able to fashion together a new table using a conditional that matches one table column to another.

SQL - Left Join

A Left join returns all rows of the left of the conditional even if there is no right column to match.

SQL Code:SELECT employees.Lastname, employees.Firstname, invoices.Sale, invoices.PriceFROM employeesLEFT JOIN invoicesON employees.id = invoices.EmployeeID




Hicks Freddy

Harris Joel



This would be a great way to track sales per person per day if the invoice table had a date field as well.

SQL - Right Join

A right join will display rows on the right side of the conditional that may or may not have a match.

SQL Code:SELECT employees.Lastname, employees.Firstname, invoices.Sale, invoices.PriceFROM employeesRIGHT JOIN invoicesON employees.id = invoices.EmployeeID





HOT DOG 1.99


LG SFT DRK 1.49

This would happen generally if perhaps nobody recieved credit for the sale or the sale was credited to the store by default.

SQL - Union

The union clause places two serarate queries together forming one table. A union works best when using two tables with similar columns because each cloumn must have the same data type.

Say we had another table called employees2 with the names and information of employees from our second store. With 2 queries, we can combine the tables into a list of all employees.

SQL Code:SELECT * FROM employeesUNIONSELECT * FROM employees2;




9 Hicks Freddy crew

10 Harris Joel crew


101 Yazzow Jim crew

102 Anderson Craig crew

103 Carlson Kevin crew

104 Maines Brad crew

The result is a complete listing every employee from store 1 and 2.

The next example shows a more practical means of using a union clause. Here we will select all of our employees from both tables, and join them with our invoices table to generate a complete list of sales from both stores on a given day.

SQL Code:SELECT employees.Lastname, employees.Firstname, invoices.Sale, invoices.PriceFROM employeesINNER JOIN invoicesON employees.id = invoices.EmployeeIDUNIONSELECT employees2.Lastname, employees2.Firstname, invoices.Sale, invoices.PriceFROM employees2INNER JOIN invoicesON employees2.id = invoices.EmployeeID;




Davis Julie CK SLD 3.99

Yazzow Jim HOT DOG 1.99

Carlson Kevin LG SFT DRK 1.49

Here we combined a join query with the union clause to create one table.

SQL - Union All

Union all selects all rows from each table and combines them into a single table. The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.

SQL Code:SELECT * FROM employeesUNION ALLSELECT * FROM employees2;




9 Hicks Freddy crew

10 Harris Joel crew


101 Yazzow Jim crew

102 Anderson Craig crew

103 Carlson Kevin crew


104 Maines Brad crew

SQL Code:SELECT employees.Lastname, employees.Firstname, invoices.Sale, invoices.PriceFROM employeesINNER JOIN invoicesON employees.id = invoices.EmployeeIDUNION ALLSELECT employees2.Lastname, employees2.Firstname, invoices.Sale, invoices.PriceFROM employees2INNER JOIN invoicesON employees2.id = invoices.EmployeeID;




Davis Julie CK SLD 3.99


Yazzow Jim HOT DOG 1.99

Carlson Kevin LG SFT DRK 1.49



SQL - Subqueries

MySQL offers a very limited support for subqueries, however Oracle and DB2 fully support them. Subqueries are Select queries placed within an existing SQL statement. They may exist in any of the following types of SQL statements.

← Select ← Insert ← Update ← Delete ← Set ← Do

Subqueries are great for answering very specific questions regarding your data inside your database. For instance, as the employer you may notice employee number 101 had a great day yesterday with sales. Just given this information we can use a subquery to pull the employee lastname and first name from our database.

SQL Code:SELECT * FROM employeesWHERE id = (SELECT EmployeeID FROM invoices WHERE EmployeeID='1');

SQL Table:id Lastname Firstname Title

11 Davis Julie MANAGER

Here we have pulled our employee information from the employees table by only knowing the employee number from the invoices table.

SQL - Subquery Inserts

Subqueries can be used to pull old data from your database and insert it into new tables. For instance if we opened up a third store and we wanted to place the same manager over 3 stores we could do this by pulling the manager's information using a subquery and then inserting the records. Also note that this form of insert will insert all cases where the subquery is true, therefore several rows may or may not be inserted depending upon how your table is set up.

SQL Code:INSERT INTO employees3 (id,Lastname,Firstname,Title)(SELECT id,Lastname,Firstname,Title FROM employees WHERE Title='manager');

With complete mastery of a subqueries you can now see the power of the SQL language. The language is capable of nearly all things imaginable.

SQL - Dates

Unfortunately, every SQL platform has its own version of date functions, the few listed work in DB2, Oracle, and MySQL. Microsoft SQL Server users should skip to our SQL Datepart lesson.

SQL - Timestamp

A timestamp servers as the catch all for dates and times. Retrieving a timestamp is very simple and the result can be converted or manipulated in nearly every way imaginable.

SQL Code:SELECT CURRENT_TIMESTAMP;

Return a Timestamp:2004-06-22 10:33:11.840

Keep in mind that each platform of SQL (DB2, Oracle, SQL Server, etc...) may return dates and times which are formatted differently.

SQL - Date Functions

As we just mentioned, it is possible to breakdown timestamps into their individual pieces using any of the following date functions.

SQL Code:SELECT MONTH(CURRENT_TIMESTAMP);

Return a Month:6

SQL Code:SELECT DAY(CURRENT_TIMESTAMP);

Return a Day:22

http://www.tizag.com/sqlTutorial/sqldatepart.php

There are many more functions available, including functions to extract milliseconds, names of the months, names of each week day, etc. Each SQL platform varies in the actual naming of date functions. Here's a few c\The following is a list of other date functions available to most platforms of SQL with the exception of MS's SQL Server.

SQL Function Code:SELECT DATE(CURRENT_TIMESTAMP); - returns a date (2004-06-22)

SELECT TIME(CURRENT_TIMESTAMP); - returns the time (10:33:11.840)

SELECT DAYOFWEEK(CURRENT_TIMESTAMP); - returns a numeric value (1-7)

SELECT DAYOFMONTH(CURRENT_TIMESTAMP); - returns a day of month (1-31)

SELECT DAYOFYEAR(CURRENT_TIMESTAMP); - returns the day of the year (1-365)

SELECT MONTHNAME(CURRENT_TIMESTAMP); - returns the month name (January - December

SELECT DAYNAME(CURRENT_TIMESTAMP); - returns the name of the day (Sunday - Saturday)

SELECT WEEK(CURRENT_TIMESTAMP); - returns number of the week (1-53)

Timestamps are often the easiest to work with, but we certainly are not limited to using only the current_timestamp as our parameter. We can send any date, time, or timestamp to the function which then returns our result.

SQL Code:SELECT MONTHNAME('2004-11-27');

Return a Month Name:MONTHNAME('2004-11-27')

November

Date functions can also be performed on table columns similarly to numeric and mathematical functions such as SUM() or AVG().

SQL Code:SELECT DAYOFYEAR(column_name) FROM table_name WHERE name = 'Joe';

SQL will return a numeric result from 1 - 365 representing the day of the year that Joe's record was created/inserted.

We can expand this concept one step further with the use of a subquery. Say we have a table with a column named timestamp. In this table column are timestamps of when each record was entered and we would like to know what was the numeric day of the year that a record was entered.

SQL Code:SELECT DAYOFYEAR((SELECT DATE(timestamp) FROM employees WHERE name = 'James Bond'));

Above you can see how it is possible to combine several date functions as well as a subquery to return very specific information about Mr. James Bond.

SQL - Inserting Date Data

Date data exists as numbers, strings, and timestamps. Built into most platforms are several date column types such as DATE or TIMESTAMP. By setting the default value to the current date or timestamp, the table column will automatically be filled with a current date/timestamp as each record is inserted.

Here's the code to add a timestamp column to an existing table.

SQL Code:ALTER TABLE `orders` ADD `order_date` TIMESTAMP DEFAULT CURRENT_TIMESTAMP NOT NULL;

Now each time an order is placed in our make believe business, a timestamp of that order is also recorded.

A date or timestamp table column will only allow date data types to be inserted as values so be sure to convert any strings or numbers to dates and timestamps before trying to insert them.

SQL - Datepart()

Microsoft's SQL Server takes a little different approach to working with dates. It is still possible to extract individual parts of a timestamp and several other functions also work as outlined in SQL - Date.

The difference is that SQL Server uses one main function as oppose to several different functions ie the DATEPART() function. Datepart() requires two parameters, a part argument and a date argument. By part we mean year, day of the year, day of the week, etc. Let's look at an example.

SQL Code:SELECT DATEPART(week, '2005-12-31');

Return the Week Number:53

Here we have successfully pulled the "week number" from existing date. SQL's CURRENT_DATE function could be also be substituted or a string value representing the year ('Dec 31, 2005').

http://www.tizag.com/sqlTutorial/sqldate.php

The following table lists the datepart() arguments that can be extracted from any given date, time, or timestamp.

part Description Abbreviation(s)

year returns a 4 digit year yy,yyyy

quarter returns a value 1-4 representing the year's quarter qq,q

month returns the numeric equivalent month (1-12) mm,m

dayofyear returns the day of the year (1-365) dy,y

day returns the day of the month (1-31) dd,d

week returns the number of the week (1-53) wk,ww

weekday returns the number of the day of the week (1-7) dw

hour returns the number of hours for a given time hh

minute returns the number of minutes from a given time mi,n

second returns the number of seconds in a given time ss,s

millisecond returns the number of milliseconds in a given time ms

SQL - Case

In SQL case works with either the select or update clauses. It provides when-then-else functionality (WHEN this_happens THEN do_this) also known as conditional statements.

SQL - CASE (select)

For this concept, visualize the following table.

employee_name admin

Ted 0

Terry 0

Trish 1

As you can see, we have a list of a few employees and then an admin column with boolean (true/false) values for their administration rights (admin). A zero means they are not an admin a 1 mean they are.

Using case logic, we can present this information in a better form.

SQL Code:SELECT employee_name CASE admin WHEN 1 THEN 'yes' ELSE 'no' END 'admin'FROM employees;

SQL Case:employee_name admin

Ted no

Terry no

Trish yes

In short all we really did is replace 1's and 0's with the words 'yes' and 'no'.

SQL - Case (update)

Case functions additionally allow for the updating of records within your table. For example, we could update the prices of items in our online store, but more importantly we could update very specific records because of the conditional logic allowed by case.

Let's use the following table for this next example.

item quantity price

goldfish 12 1.00

guppy 24 0.50

blow fish 1 5.00

Let's say we wanted to have a sale to clean out some of our overstock. We'll go ahead and take 25% off of all our items that we currently have 20 or more of (>20). Then we'll take 10% off the items that we have 10-20 of (between 10 and 20) and everything else we will discount only 5%.

SQL Code:UPDATE inventory SET price = price * CASE WHEN quantity > 20 THEN 0.75 WHEN quantity BETWEEN 10 AND 20 THEN 0.90 ELSE 0.95END;

SQL Case:item quantity price

goldfish 12 .9

guppy 24 0.375

blow fish 1 4.75

Each price has automatically been reduced by the appropriate percentages.

SQL - Historical Information

In the 60's database software required the use of complex mainframe machines that were difficult to maintain and run. Information technologists worked around the clock monitoring, updated, and manually uncorrupting these machines.

Each mainframe ran different software from different manufacturers. IBM pulled ahead in software development internationally with efforts of software aimed at database management. The problem was that each mainframe ran a different type of "language".

Enter SQL. The new standard for any database program: Structured Query Language. SQL bridged the barriers between mainframes and allowed large corporations to network their efforts. SQL was introduced in the 1970's and quickly gained international popularity. SQL allows a programmer to tell a program exactly what data to retrieve and how to display it.

SQL - Under Development

Software at the time was still underdeveloped and several continuous problems plagued database storage. Transactions happening at the same time were often combined, mixed together, or even lost altogether. For instance, say two individuals place make a deposit at exactly the same time from two different locations, often times the software was unable to cope with this and their bank transactions could be completely switched. Suzy deposited $10,000 while Joe withdrew $15. Because of the faulty software it was not uncommon to see Joe's withdrawal post on Suzy's account while Suzy's deposit is posted on Joe's account.

Along came Relational Database Management Systems (RDBMS). This software is still being used and is quite powerful. Relational databases allow the developer to build relationships between databases and tables. This provides tremendous opportunities for data management and is still the favorite software used today. MySQL, SQL Server, DB2, and Oracle are all RDBMSs and each have a substantial hold in the market share to this day.

SQL - Structure

The importance of building your tables correctly could be considered its own artform. Psychologists have even gone as far to say that those with the ability to organize their thoughts and concepts well, tend to be absolutely brilliant (ie Mozart, Einstein, DaVinci). A database is only slightly different than the human brain and clear organization is a must, especially if many people will be using the same database.

Designing the right database has a lot to do with what you need to accomplish. A database with thousands and thousands or even millions of records requires a completely different solution than a database that will only have a few hundred records. Think about the future and plan ahead.

SQL - Table Relationships

The key to a large database is having tables with material that can relate to one another. A great example is the employee ID. Ever wonder or joke about being just another number to the company. A lot of that has to do with the way they set up their database. A large company is going to have a weekly payroll and a personal information table. They might be set up like the following:

personal_info emp_id last_name first_name

0056 Mynce George

0097 Jenkins Fred

weekly_payroll emp_id hours rateofpay

0056 37.5 27.00

0097 44.5 22.25

This set up proves useful in several ways. The tables are smaller, more compact, and precise allowing for maximum access speeds with each query. We can also moderate who has access to each

table, meaning that our accountant can have access to the payroll table but not the personal information table which will eliminate security risks.

Relationship tables are the choice for high-traffic databases. There are countless security advantages, faster query returns, and complete view customization (more on this later).

SQL - Identifiers, Think SQL

As with any programming language, there are a few terms unique to that language.

An Identifier is essentially a name of a database, table, or table column. As the creator of the database you are free to identify these objects as you please, we merely suggest you keep these guidelines in mind when doing so.

← Develop your own unique naming scheme. -- Use terms that relate to one another and will be easy to recognize apart from your code.

← Be conscious of how long your names become. -- Especially be aware when the time comes to name your columns.

← Avoid names without meaning. -- Develop a working scheme that also has meaning behind the names.

← Be consistent. -- If you capitalize one table name, capitalize each table name, if you choose to use abbreviations make sure they do not have double meanings or ambiguous meaning.

Develop a clear, concise schema and stick to it as your database develops.

SQL - Literals

Literal is a term for data types such as strings, numbers, or boolean values in SQL. These values are not named by you the programmer they just exist. Strings, numbers, and boolean values are literals.

Literal Breakdown:string literals'This is a string value''5893 Moon River Dr.'

number literals823-4.53.387920

boolean literalsTRUEFALSE10

SQL - Keywords and Reserved Terms

A reserved term is a word used in the actual processing of data. A few lessons down the road you will be introduced to terms called clauses. These clauses are commands issued to the SQL program and usually involve some sort of data manipulation. For instance the clause "CREATE" is used to

create tables and databases therefore CREATE is a reserved term. Functions are also reserved terms. A simple function is the SUM() function. This function will simply sum up all the numbers of one of your table columns and return the result. Since SUM is used as a function by SQL it is a reserved term. Reserved terms should not be used as identifiers.

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