Getting Started with SQL

Chapter 2. Databases

What Is a Database?

In the broadest definition, a database is anything that collects and organizes data. A spreadsheet holding customer bookings is a database, and so is a plain-text file containing flight schedule data. Plain-text data itself can be stored in a variety of formats, including XML and CSV.

Professionally, however, when one refers to a “database” they likely are referring to a relational database management system (RDBMS). This term may sound technical and intimidating, but an RDBMS is simply a type of database that holds one or more tables that may have relationships to each other.

Exploring Relational Databases

A table should be a familiar concept. It has columns and rows to store data, much like a spreadsheet. These tables can have relationships to each other, such as an ORDER table that refers to a CUSTOMER table for customer information.

For example, suppose we have an ORDER table with a field called CUSTOMER_ID (Figure 2-1).

Figure 2-1. An ORDER table with a CUSTOMER_ID

We can reasonably expect there to be another table, maybe called CUSTOMER (Figure 2-2), which holds the customer information for each CUSTOMER_ID.

Figure 2-2. A CUSTOMER table

When we go through the ORDER table, we can use the CUSTOMER_ID to look up the customer information ...