creating relationships between tables (sql) by hubspot-directory.blogspot.com

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