Using subqueries

In this section, we will talk about subqueries. Subqueries can be described as nested queries – we can nest a query inside another query using parentheses. Subqueries can return a single value or a recordset, just like regular queries. We will start by introducing subqueries using the IN/NOT IN operator.

Subqueries and the IN/NOT IN condition

Let’s start with the IN operator; we can use the IN operator inside a where clause instead of using multiple OR conditions. For example, if you wanted to search for all categories that have the value pk=1 or the value pk=2, you would have to perform the following statement:

forumdb=> select * from categories where pk=1 or pk=2;
 pk |  title   |         description     
----+----------+------------------------------
  1 | Database | Database related discussions
  2 | Unix     | Unix and Linux discussions
(2 rows)

Another way to reach the same outcome is the following:

forumdb=> select ...