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Creating Relationships between tables (SQL) by hubspot-directory.blogspot.comTRANSCRIPT
Chapter 6
Creating Relationships Between Tables
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In this chapter, you will learn:
The basic commands and functions of SQLHow to use SQL for data administration (to
create tables)How to use SQL for data manipulation (to
add, modify, delete, and retrieve data)How to use SQL to query a database to
extract useful information
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Introduction to SQLSQL functions fit into two broad categories:
Data definition languageSQL includes commands to:
Create database objects, such as tables, indexes, and views
Define access rights to those database objectsData manipulation language
Includes commands to insert, update, delete, and retrieve data within database tables
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Introduction to SQL (continued)
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Introduction to SQL (continued)
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Introduction to SQL (continued)
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Data Definition CommandsExamine simple database model and
database tables that will form basis for many SQL examples
Understand data environment
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The Database Model
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Creating the DatabaseFollowing two tasks must be completed:
Create database structure Create tables that will hold end-user data
First task:RDBMS creates physical files that will hold
databaseTends to differ substantially from one RDBMS
to another
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The Database Schema
Authentication › Process through which DBMS verifies that
only registered users are able to access database
› Log on to RDBMS using user ID and password created by database administrator
Schema› Group of database objects—such as tables
and indexes—that are related to each other
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Data TypesData type selection is usually dictated by
nature of data and by intended usePay close attention to expected use of
attributes for sorting and data retrieval purposes
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Data Types (continued)
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Creating Table StructuresUse one line per column (attribute)
definitionUse spaces to line up attribute
characteristics and constraintsTable and attribute names are capitalizedNOT NULL specification UNIQUE specification
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Creating Table Structures (cont)
Primary key attributes contain both a NOT NULL and a UNIQUE specification
RDBMS will automatically enforce referential integrity for foreign keys
Command sequence ends with semicolon
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SQL ConstraintsNOT NULL constraint
Ensures that column does not accept nullsUNIQUE constraint
Ensures that all values in column are uniqueDEFAULT constraint
Assigns value to attribute when a new row is added to table
CHECK constraint Validates data when attribute value is entered
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Data Manipulation Commands
Adding table rowsSaving table changesListing table rowsUpdating table rowsRestoring table contentsDeleting table rowsInserting table rows with a select subquery
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Adding Table RowsINSERT
Used to enter data into tableSyntax:
INSERT INTO columnnameVALUES (value1, value2, … , valuen);
Example: INSERT INTO PRODUCT VALUES (‘11QER’,’Power painter’, ‘12/3/2000’, 2121344);
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Adding Table Rows (continued)When entering values, notice that:
Row contents are entered between parentheses
Character and date values are entered between apostrophes
Numerical entries are not enclosed in apostrophes
Attribute entries are separated by commasA value is required for each column
Use NULL for unknown values
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Saving Table Changes
Changes made to table contents are not physically saved on disk until, one of the following occurs:› Database is closed› Program is closed› COMMIT command is used
Syntax:› COMMIT [WORK];
Will permanently save any changes made to any table in the database
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Listing Table Rows
SELECT › Used to list contents of table› Syntax:
SELECT columnlistFROM tablename;
Columnlist represents one or more attributes, separated by commas
Asterisk (*) can be used as wildcard character to list all attributes
Example: SELECT* FROM PRODUCT;
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Listing Table Rows (continued)
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Updating Table RowsUPDATE
Modify data in a tableSyntax:
UPDATE tablenameSET columnname = expression [, columname = expression][WHERE conditionlist];
If more than one attribute is to be updated in row, separate corrections with commas
Example: UPDATE PRODUCT SET P_INDATE = ‘02/18/2012’, P-PRICE = 15.99, P_MIN =
10 WHERE P_CODE = ‘11QER’;
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Restoring Table Contents
ROLLBACK› Used to restore database to its previous
condition› Only applicable if COMMIT command has not
been used to permanently store changes in database
Syntax:› ROLLBACK;
COMMIT and ROLLBACK only work with data manipulation commands that are used to add, modify, or delete table rows
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Deleting Table RowsDELETE
Deletes a table rowSyntax:
DELETE FROM tablename[WHERE conditionlist ];
WHERE condition is optionalIf WHERE condition is not specified, all rows
from specified table will be deletedExample: DELETE FROM PRODUCT
WHERE P_CODE = ‘11QER’;
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Inserting Table Rows with a Select Subquery
INSERT› Inserts multiple rows from another table
(source)› Uses SELECT subquery
Query that is embedded (or nested) inside another query
Executed first
› Syntax: INSERT INTO tablename SELECT columnlist FROM
tablename;
› Example: INSERT INTO MySalesReason SELECT SalesReasonID, Name, ModifiedDate FROM AdventureWorks2008
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Selecting Rows with Conditional Restrictions
Select partial table contents by placing restrictions on rows to be included in outputAdd conditional restrictions to SELECT
statement, using WHERE clauseSyntax:
SELECT columnlistFROM tablelist[ WHERE conditionlist ] ;
Example SELECT P_DESCRIPT, P_INDATE, P_PRICE FROM PRODUCT WHERE V_CODE = 21344;
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Selecting Rows with Conditional Restrictions (continued)
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Selecting Rows with Conditional Restrictions
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Selecting Rows with Conditional Restrictions (continued)
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Selecting Rows with Conditional Restrictions (continued)
SELECT P_DESCRIPT, P_INDATE, P_PRICE, V_CODEFROM PRODUCTWHERE V_CODE <> 12344;
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Selecting Rows with Conditional Restrictions (continued)
SELECT P_DESCRIPT, P_QOH, P_MIN, P_PRICEFROM PRODUCTWHERE P_PRICE <= 10;
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Selecting Rows with Conditional Restrictions (continued)
SELECT P_DESCRIPT, P_QOH, P_MIN, P_PRICE, P_QOH * P_PRICEFROM PRODUCT;
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Arithmetic Operators: The Rule of Precedence
Perform operations within parenthesesPerform power operationsPerform multiplications and divisionsPerform additions and subtractions
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Arithmetic Operators: The Rule of Precedence (continued)
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Logical Operators: AND, OR, and NOT
SELECT P_DESCRIPT, P_INDATE, P_PRICE, V_CODEFROM PRODUCTWHERE V_CODE = 21344OR V_CODE = 24288;
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Logical Operators: AND, OR, and NOT (cont)
SELECT P_DESCRIPT, P_INDATE, P_PRICE, V_CODEFROM PRODUCTWHERE P_PRICE < 50AND P_DATE >= ’01/20/2006’;
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Logical Operators: AND, OR, and NOT (cont)
SELECT P_DESCRIPT, P_INDATE, P_PRICE, V_CODEFROM PRODUCTWHERE (P_PRICE < 50 AND P_INDATE > ’12/15/2006’)OR V_CODE = 24288;
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Special Operators
BETWEENUsed to check whether attribute value is within a
rangeIS NULL
Used to check whether attribute value is nullLIKE
Used to check whether attribute value matches given string pattern
INUsed to check whether attribute value matches
any value within a value listEXISTS
Used to check if sub query returns any rows
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Special Operators
Example:SELECT*FROM PRODUCTWHERE P_PRICE BETWEEN 50.00 AND 100.00;
SELECT P_CODE, P_DESCRIPTFROM PRODUCTWHERE P_MIN IS NULL;
SELECT V_NAME, V_CONTACT, V_AREACODE, V_PHONEFROM VENDORWHERE V_CONTACT LIKE ‘Smith%’; - NOT LIKE if do not
want any Smith
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EXAMPLES SELECT first_name, last_name, subject
FROM student_details WHERE subject = ‘Maths’ OR subject = ‘Science’;
SELECT first_name, last_name, age FROM student_details WHERE age >= 10 AND age <= 15;
SELECT first_name, last_name, games FROM student_details WHERE NOT games = ‘Football’;
SELECT first_name, last_name, age, games FROM student_details WHERE age >= 10 AND age <= 15 OR NOT games = ‘Football’;
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Advanced Data Definition Commands
All changes in table structure are made by using ALTER commandFollowed by keyword that produces specific
change Following three options are available:
ADDMODIFYDROP
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Changing a Column’s Data Type
ALTER can be used to change data typeSome RDBMSs (such as Oracle) do not
permit changes to data types unless column to be changed is empty
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Changing a Column’s Data Characteristics
Use ALTER to change data characteristicsIf column to be changed already contains
data, changes in column’s characteristics are permitted if those changes do not alter the data type
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Adding a ColumnUse ALTER to add column
Do not include the NOT NULL clause for new column
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Dropping a ColumnUse ALTER to drop column
Some RDBMSs impose restrictions on the deletion of an attribute
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Adding Primary and Foreign Key Designations
When table is copied, integrity rules do not copy, so primary and foreign keys need to be manually defined on new table
User ALTER TABLE commandSyntax:
ALTER TABLE tablename ADD PRIMARY KEY(fieldname);
For foreign key, use FOREIGN KEY in place of PRIMARY KEY
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Deleting a Table from the Database
DROPDeletes table from databaseSyntax:
DROP TABLE tablename;
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Advanced Select QueriesSQL provides useful functions that can:
CountFind minimum and maximum valuesCalculate averages
SQL allows user to limit queries to only those entries having no duplicates or entries whose duplicates may be grouped
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Aggregate Functions
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Aggregate Functions (continued)
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Aggregate Functions (continued)
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Aggregate Functions (continued)
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Aggregate Functions (continued)
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Grouping Data
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Grouping Data (continued)
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Grouping Data (continued)
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Joining Database TablesAbility to combine (join) tables on common
attributes is most important distinction between relational database and other databases
Join is performed when data are retrieved from more than one table at a time
Join is generally composed of an equality comparison between foreign key and primary key of related tables
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Joining Database Tables (continued)
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Joining Database Tables (continued)
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Joining Database Tables (continued)
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Joining Tables with an AliasAlias can be used to identify source tableAny legal table name can be used as aliasAdd alias after table name in FROM clause
FROM tablename alias
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Recursive Joins
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Recursive Joins (continued)
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Outer Joins
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Outer Joins (continued)
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SummarySQL commands can be divided into two
overall categories: Data definition language commands Data manipulation language commands
The ANSI standard data types are supported by all RDBMS vendors in different ways
Basic data definition commands allow you to create tables, indexes, and views
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Summary (continued)DML commands allow you to add, modify, and
delete rows from tablesThe basic DML commands are SELECT, INSERT,
UPDATE, DELETE, COMMIT, and ROLLBACKINSERT command is used to add new rows to
tablesSELECT statement is main data retrieval
command in SQL
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Summary (continued)Many SQL constraints can be used with
columnsThe column list represents one or more
column names separated by commasWHERE clause can be used with SELECT,
UPDATE, and DELETE statements to restrict rows affected by the DDL command
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Summary (continued)Aggregate functions
Special functions that perform arithmetic computations over a set of rows
ORDER BY clauseUsed to sort output of SELECT statementCan sort by one or more columns and use
either an ascending or descending orderJoin output of multiple tables with SELECT
statement
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Summary (continued)Natural join uses join condition to match
only rows with equal values in specified columns
Right outer join and left outer join used to select rows that have no matching values in other related table