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Single Table Queries

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Objectives

SELECT, WHERE AND / OR / NOT conditions Computed columns LIKE, IN, BETWEEN operators ORDER BY, GROUP BY, HAVING SUM, COUNT, AVG, MIN, MAX, DISTINCT

functions Subqueries and Nulls

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Simple Queries

Query - a question represented in a way that the DBMS can understand

SELECT clause Followed by the columns to be included in the query.

FROM clause followed by name of the table that contains the data to query

WHERE clause (optional) followed by conditions that apply to the data to be retrieved

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Note

Generally SQL is not case sensitive Exception:

Values within quotation marks Use the correct case for these values.

Example: WHERE LAST ‘adams’ will not select any rows if the

stored value is “Adams”

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Compound Conditions

Formed by connecting two or more simple conditions Uses AND, OR, and NOT operators

AND: all conditions must be true OR: any one of conditions is true NOT: reverses the truth of the original condition

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AND / OR / NOT Conditions

List the description of every part that is in warehouse number 3 and that has more than 100 units on hand

List the description of every part that is in warehouse number 3 or that has more than 100 units on hand

List the description of every part that is not in warehouse number 3

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Use of BETWEEN

BETWEEN operator Not an essential feature Can arrive at same answer without it using AND Does make certain SELECT commands simpler

List the customer number, last name, first name, and balance for every customer whose balance is between $500 and $1000

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Computed Columns

Computed columns Do not exist in the database Can be computed using data in existing columns Use arithmetic operators

Find the customer number, last name, first name, and available credit for every customer who has a credit limit of at least $1,500

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Use of LIKE

LIKE operator is used when exact matches will not work Use LIKE with a wildcard symbol Percent symbol (%)

represents any collection of characters ‘%Pine%’

Underscore (_) Represents any individual character ‘T_m’

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Use of IN

The IN clause provides concise way of phrasing certain conditions

List the customer number, last name, and first name for every customer with a credit limit of $1,000, $1,500, or $2,000

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Use of ORDER BY

Use the ORDER BY command to list data in a specific order

The column on which data is to be sorted is called a sort key or simply key

List the customer number, last name, first name, and balance of every customer

Order the output in ascending (increasing) order by balance

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Sorting with Multiple Keys

When sorting more than one column the more important column is called the major key (or

primary sort key) the less important column is called the minor key (or

secondary sort key)

List keys in the order of importance in the ORDER BY clause

Sort descending by using the DESC operator

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Sorting with Multiple Keys

List the customer number, last name, first name, and credit limit of every customer, ordered by credit liming in descending order and by last name within credit limit

The output should be sorted by credit limit in descending order

Sort the output by last name within each group of customers with the same credit limit

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Using Functions

SQL has functions to calculate Sums Averages Counts Maximum values Minimum values

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Use of COUNT Function

Counts the number of rows in a table The specific row to be counted is not important

because each count should provide the same answer Most implementations of SQL allow the use of the

asterisk (*) to represent any column

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Use of the SUM Function

The SUM function is used to calculate totals The column to be totaled must be specified The column to be totaled must be numeric Find the number of customers and the total of their

balance

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Using AVG, MAX, and MIN

AVG, MAX and MIN functions are similar to the SUM

SUM, AVG, MAX and MIN functions ignore (eliminate) null values Null values can cause strange results when calculated

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Use of DISTINCT

DISTINCT operator is not a function Useful when used in conjunction with COUNT

function Find the customer number of every customer who

currently has an open order (List each customer only once)

Count the number of customers who currently have open orders

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Nesting Queries

Sometimes obtaining the results you need is a two-step process (or more).

What is the largest credit limit given to any customer of sales rep 06?

Display the customer number, last name, and first name of every customer in the Premiere Products database who has the above determined credit limit

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Subqueries

It is possible to place one query inside another Inner query is called a subquery and it is evaluated first Outer query can use the results of the subquery to find its

results Rewrite the previous query in one step List the customer number, first name, last name, and balance

for every customer whose balance is greater than the average balance

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Note

SQL will not allow the use of the condition BALANCE > AVG(BALANCE) in the WHERE clause

A subquery must be used to obtain the average balance

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Grouping

Grouping creates groups of rows that share some common characteristics

Calculations are performed for the entire group GROUP BY command allows data to be grouped in

a particular order Statistics are calculated on the groups

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HAVING vs. WHERE

WHERE clause limits rows HAVING clause limits groups

List each credit limit and the total number of customers of sales rep 03 who have this limit

Repeat above, but list only those credit limits held by more than one customer

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Null Values

List the customer number, last name, and first name of every customer whose street value is null (unknown)

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Summary

The basic form of a SQL command is SELECT-FROM. Specify the columns to be listed after the word SELECT (or type * to select all columns), and then specify the table name that contains these columns after the word FROM. Optionally, you can include conditions after the word WHERE

Simple conditions are written in the form: column name, comparison operator, column name or value. Simple conditions can involve any of the comparison operators: =, >, >=, <, <=, or <> or != (not equal to).

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Summary

You can form compound conditions by combining in simple conditions, using the operators AND, OR, or NOT.

Use the BETWEEN operator to indicate a range of values in a condition

Use computed columns in SQL commands by using arithmetic operators and writing the computation in place of a column name

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Summary

To check for a value in a character column that is similar to a particular string of characters, use the LIKE clause

The % wildcard represents any collection of characters The _ wildcard represents any single character

To check whether a column contains one of a particular set of values, use the IN clause

Use the ORDER BY clause to sort data List sort keys in order of importance To sort in descending order, follow the sort key with DESC

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Summary

SQL contains the functions COUNT, SUM, AVG, MAX, AND MIN

To avoid duplicates, either when listing or counting values, precede the column name with the DISTINCT operator

When one SQL query is placed inside another, it is called a nested query

The inner query, called a subquery, is evaluated first.

Use the GROUP BY clause to group data

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Summary

Use the HAVING clause to restrict the output to certain groups

Use the phrase IS NULL in the WHERE clause to find rows containing a null value in some column