Operations on tables

Because a database is really a collection of tables, database operations are operations on tables. The object-relational model supports three fundamental operations: selection, projection, and joining. The following figure shows the selection and projection operations. (All three operations are defined in detail, with many examples, in the following topics.)

Figure 1. Illustration of selection and projection

When you select data from a table, you are choosing certain rows and ignoring others. For example, you can query the stock table by asking the database management system to, "Select all rows in which the manufacturer code is HSK and the unit price is between 200.00 and 300.00."

When you project from a table, you are choosing certain columns and ignoring others. For example, you can query the stock table by asking the database management system to "project the stock_num, unit_descr, and unit_price columns."

A table contains information about only one entity; when you want information about multiple entities, you must join their tables. You can join tables in many ways. For more information about join operations, refer to Compose advanced SELECT statements.