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 , DML and DCL. The four SQL statements form the basis of DML: SELECT, INSERT, UPDATE and DELETE. They were described in this chapter.

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

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