SQL is used to interact with the database: create and maintain the data structures, put data into the database, change it, retrieve it, and delete it. SQL has commands related to DDL (Data Definition Language), DML (Data Manipulation Language), and DCL (Data Control Language). The four SQL statements form the basis of DML: SELECT, INSERT, UPDATE, and DELETE, which were described in this chapter.

The SELECT statement was examined in detail to explain SQL concepts such as grouping and filtering to show SQL expressions and conditions, and to explain how to use subqueries. Additionally, some relational algebra topics were covered in the section about joining tables.

In the next chapters, the more complicated topics will be covered--some advanced SQL features and techniques and the programming language PL/pgSQL, which can be used to implement functions in PostgreSQL.