Relational Databases

A relational database is a logical representation of data that allows the data to be accessed without consideration of its physical structure. A relational database stores data in tables. Figure 25.1 illustrates a sample table that might be used in a personnel system. The table name is Employee, and its primary purpose is to store the attributes of an employee. Tables are composed of rows, and rows are composed of columns in which values are stored. This table consists of six rows. The Number column of each row in this table is the table's primary keya column (or group of columns) in a table with a unique value that cannot be duplicated in other rows. This guarantees that each row can be identified by its primary key. Good examples of primary key columns are a Social Security number, an employee ID number and a part number in an inventory system, as values in each of these columns are guaranteed to be unique. The rows in Fig. 25.1 are displayed in order by primary key. In this case, the rows are listed in increasing order, but we could also use decreasing order. Rows in tables are not guaranteed to be stored in any particular order. As we will demonstrate in an upcoming example, programs can specify ordering criteria when requesting data from a database.

Figure 25.1. Employee table sample data.

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Each column represents a different data attribute. Rows are normally unique (by primary key) within a table, but particular column values may be duplicated between rows. For example, three different rows in the Employee table's Department column contain number 413.

Different users of a database are often interested in different data and different relationships among the data. Most users require only subsets of the rows and columns. To obtain these subsets, we use queries to specify which data to select from a table. Programmers use SQL to define complex queries that select data from a table. For example, we might select data from the Employee table to create a result that shows where each department is located. This result is shown in Fig. 25.2. SQL queries are discussed in Section 25.4.