A subquery is a query within a query. In other words, a subquery is a SQL query nested inside a larger query. It may occur in a SELECT, FROM, or WHERE clause. In PostgreSQL, a subquery can be nested inside a SELECT, INSERT, UPDATE, DELETE, SET, or DO statement or inside another subquery. It is usually added within the WHERE clause of another SQL SELECT statement. You can use comparison operators, such as >, <, or =. Comparison operators can also be a multiple-row operator, such as IN, ANY, SOME, or ALL. It can be treated as an inner query that is an SQL query placed as a part of another query called as outer query. The inner query is executed before its parent query so that the results of the inner query can be passed to the outer query.

The following statement illustrates the subquery syntax:

SELECT column list
FROM table 
WHERE table.columnname expr_operator
(SELECT column FROM table)

The query inside the brackets is called the inner query. The query that contains the subquery is called the outer query.

PostgreSQL executes the query that contains a subquery in the following sequence:

Let's consider an example where you want to find employee_id, first_name, last_name, and salary for employees whose salary is higher than the average salary throughout the company.

We can do this in two steps:

        SELECT avg(salary) from employee;

        Result: 25000
        SELECT employee_id,first_name,last_name,salary
        FROM employee
        WHERE salary > 25000;

This does seem rather inelegant. What we really want to do is pass the result of the first query straight into the second query without needing to remember it, and type it back for a second query.

The solution is to use a subquery. We put the first query in brackets, and use it as part of

a WHERE clause to the second query, as follows:

SELECT employee_id,first_name,last_name,salary
FROM employee
WHERE salary > (Select avg(salary) from employee);

PostgreSQL runs the query in brackets first, that is, the average of salary. After getting the answer, it then runs the outer query, substituting the answer from the inner query, and tries to find the employees whose salary is higher than the average.