A Guide to SQL, Seventh Edition

A Guide to SQL, Seventh Edition

Objectives

Understand the concepts and terminology associated with relational databases

Create and run SQL commands in Oracle, Microsoft Access, and MySQL

Create tables using SQL


Objectives

Identify and use data types to define columns in SQL tables

Understand and use nulls

Add rows to tables

Describe a table’s layout using SQL


Introduction

Databases are found at school libraries, on the Internet, and other places where you retrieve data on a computer

Learn concepts and terminology associated with relational model for database management

Learn how to create a database by describing and defining tables and columns


Introduction

SQL (Structured Query Language) is a widely used language for retrieving and manipulating data

SQL was developed in the mid-1970’s for IBM under the name SEQUEL

Renamed SQL in 1980

Most DBMSs use a version of SQL as their data manipulation language


Introduction

You will learn to assign data types to columns and the use of null values

You will learn how to load a database by creating tables and adding data to them

You will learn how to describe a table’s layout using SQL


Relational Databases

A relational database is a collection of tables

Tables are called relations

This term is the basis for the name relational database




Entities, Attributes, and Relationships

An entity is a person, place, object, event, or idea for which you want to store and process data

Premier Product’s entities are customers, orders, parts, and sales reps

Entities for a school would be students, faculty, and classes



Attribute is a character or property of an entity

Premiere Products attributes for the entity “customer” are customer name, street, city, and so on

Also called a field or column in many database systems



A relationship is the association between entities

At Premiere Products, there is a relationship between customers and sales reps

One-to-many relationship

• Each sales rep has many customers

• Each customer has only one sales rep



Premiere Products database

One table for sales reps, one for customers, and so on for each entity

Attributes become columns in each table

Relationship is represented by using common columns in two or more tables



Definitions

A relation is a two-dimensional table in which:

Entries are single-valued

Each column has a distinct name

All values of the same attribute

Order of columns and rows is immaterial

Each row is distinct


Definitions

A relational database is a collection of relations

A row in a table is called a record or a tuple

Multiple entries in one position in a table are called a repeating group


Shorthand Representation

After the name of the table, the columns are listed within a set of parentheses

REP (Rep_Num, Last_Name, First_Name, Street, City, State, ZIP, Commission_Rate)

CUSTOMER (Customer_Num, Customer_Name, Street, City, State, ZIP, Balance, Credit_Limit, Rep_Num)


Shorthand Representation

Not case-sensitive except when inserting character values in a table

Use a period to separate the table and column name to avoid confusion

The primary key uniquely identifies a row in a table

Indicate primary key by underlining the column or collection of columns


Database Creation

Describe the layout of each table in the database

Use CREATE TABLE command

TABLE is followed by the table name

Follow this with the names and data types of the columns in the table

Data types define type and size of data


Table and Column Name Restrictions

Names cannot exceed 18 characters

In Oracle, can be up to 30 characters in length

Must start with a letter

Can contain letters, numbers, and underscores (_)

Cannot contain spaces


Create Table Command



Table name is REP

Contains nine columns: REP_NUM, LAST_NAME, FIRST_NAME, STREET, CITY, STATE, ZIP, COMMISSION, RATE

REP_NUM is limited to two characters and is the primary key

RATE column is limited to numbers, three digits with two decimal places



Commands are free-format; no rules stating specific words in specific positions

Hit enter to move to the next line in a command

Indicate the end of a command by typing a semicolon


Running SQL Commands

Programs and individual preference determine method for running command

Oracle 10g use SQL*Plus or SQL*Plus Worksheet

Create queries in SQL view in Microsoft Access

Commands are typed at prompt in MySQL






Editing SQL Commands

Edit commands as you would in a word processor when using Oracle SQL*Plus Worksheet or Access SQL

After making edits, click the Execute or Run button

With Oracle SQL*Plus or MySQL the on-screen command must be edited to change it


Editing Oracle SQL*Plus

In Oracle SQL*Plus you must edit commands one line at a time

The most recent command is stored in the command buffer (the buffer)

Edit the command in the buffer by using editing commands





Editing MySQL Commands

Most recent command is stored in memory area called statement history

Edit command in statement history by using specific editing commands




Editing MySQL Commands

Press Up arrow key to go to top line

Hit Enter key to move to next line if line is correct

Use Right and Left arrow keys to move to point needing correction

When line is correct hit Enter key

If Enter is not hit on a line, that line will not be part of the revised command


Dropping a Table

Another way to correct errors is to drop (delete) a table and start over

Useful when table is created before errors are discovered

Command is followed by the table to be dropped and a semicolon

Data is deleted when a table is dropped


Data Types

For each column, the type of data must be defined

Actual data types vary between SQL programs there are some common types

CHAR(n), DATE, DECIMAL(p,q), INTEGER, SMALLINT


Nulls

A null is used as special value to represent situation when actual value is not known for a column

Specify whether to allow nulls in the individual columns

Nulls should not be allowed for primary key columns


Implementation of Nulls

NOT NULL clause is used in a CREATE TABLE command to exclude the use of nulls in a column

Default is to allow null values

If a column is defined as NOT NULL, system will reject any attempt to store a null value there


Loading a Table with Data

INSERT Command Adds rows to a table

INSERT INTO followed by the table name

VALUES command follows with specific values in parentheses

Values for character columns are in single quotation marks


The Insert Command


Modifying the INSERT Command

To add new rows it is easier to modify previous insert command and execute it

Oracle SQL*Plus Worksheet and Access SQL view, select the text, modify and execute

Oracle SQL*Plus or MySQL, edit commands manually







The INSERT Command with Nulls

Use a special format of INSERT command to enter a null value in a table

Identify the names of the columns that accept non-null values, then list only the non-null values after the VALUES command


The INSERT Command with Nulls

Enter only non-null values Precisely indicate values you are entering by

listing the columns


Viewing Table Data

SELECT command is complex

A simple version can display all the rows and columns in a table

SELECT*FROM followed by the name of the table with the data you want to view

As with other SQL commands, it ends with a semicolon


Correcting Errors in the Database

UPDATE command is used to update a value in a table

DELETE command allows you to delete a record

INSERT command allows you to add a record




Saving SQL Commands

Allows you to use commands again without retyping

Different methods for each SQL implementation you are using

Oracle SQL*Plus and SQL*Plus Worksheet use a script file

Access saves queries as objects

MySQL uses an editor to save text files


Creating the Remaining Tables

Execute appropriate CREATE TABLE and INSERT commands

Save these commands on your hard drive or a floppy disk for backup




Describing a Table

When working with a table, there may not be access to the CREATE TABLE command used for the table

Examining the table’s structure will show details about the columns

Each DBMS has a method to examine a table’s structure


Describing a Table

Oracle SQL*Plus and SQL*Plus Worksheet use DESCRIBE command

Access uses the Documenter

MySQL uses the SHOW COLUMNS command


Summary

Use the CREATE TABLE command to create tables

DROP TABLE, INSERT, SELECT, UPDATE and DELETE commands are used to edit tables

Display a table’s structure and layout for information


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A Guide to SQL, Seventh Edition

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