What Is a Relational Database?

A relational database is a collection of data arranged in tables, also known as relations. Tables are subject to the following relational operations, each of which lets you retrieve data in a specific way:

Selection, which lets you extract a subset of rows.
Projection, which lets you extract a subset of columns.
Joining, which lets you extract from more than one table at a time.

In practice, these operations frequently appear together. SQL statements that use these operations are known as queries. Three queries that use the SQL SELECT statement to illustrate selection, projection, and joining are shown in Figure 1-1.

Figure 1-1 Relational Operations

Rows and Columns

When you look at data in relational terms, you can assume several things:

Tables are arranged in rows and columns, which are like records and fields in an ordinary file.
Each column has a specific data type and size.
Each row contains one element for every column.
A column can contain NULL values if you allow it to.

Sample Database Table

The following is a portion of a database table consisting of names and account balances for an employee credit union:

Table 1-1 Employee Accounts

Last Name	First Name	Telephone	Employee Number	Balance
Harrison	Gerald	7233	2432099	142.59
Abelson	Annette	4312	3510044	2345.09
Stanley	Peter	NULL	3540011	321.98
Walters	Georgia	2554	9124772	1230.10

Notice that the third row contains a NULL value in the third column instead of a value for Telephone.

Data Types and Sizes

Each column can accept data of a specific type and size. Here is the breakdown for the sample table above:

Column Name: Data Type
Last Name: VARCHAR(15)
First Name: VARCHAR(15)
Telephone: SMALLINT
Employee Number: INTEGER
Balance: DECIMAL(10,2)

Data types are described further in chapter 2.

Using Several Tables

You can put the same data into several different tables such as the following:

Table 1-2 Employees Table

Last Name	First Name	Employee Number
Harrison	Gerald	2432099
Abelson	Annette	3510044
Stanley	Peter	3540011
Walters	Georgia	9124772

Table 1-3 Telephone Table

Last Name	First Name	Telephone
Harrison	Gerald	7233
Abelson	Annette	4312
Stanley	Peter	NULL
Walters	Georgia	2554

Table 1-4 Accounts Table

Employee Number	Account Balance
2432099	142.59
3510044	2345.09
3540011	321.98
9124772	1230.10

You decide which arrangements of data work best for you by using the processes of data analysis and database design.

In data analysis, you investigate the various ways your data can be used. In database design, you create specific table structures based on your analysis. The design phase results in a set of table descriptions, sometimes known as a schema, for your database.

Chapter 2 presents an introduction to data analysis and database design.

What Is a Relational Database?

Technical documentation

» Table of Contents

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Rows and Columns

Sample Database Table

Data Types and Sizes

Using Several Tables