What are sub-queries?

Sub queries are select statements that appear as part of another SQL statement. A sub query is often used when comparing data from different tables. It is written in the WHERE clause, and must be surrounded by brackets so that it cannot be confused with its parent query.

SELECT first_name, last_name
FROM employees
WHERE department_id=(SELECT department_id
                     FROM department
                     WHERE section_id=1112);

The above example lists the first name and last name of all the employees from section 1112. The section ID field is not in the employee table but the department ID field is. The section ID is in the department table, so this table is used to find a matching department ID that can be used to query the employee table.

SELECT first_name, last_name
FROM employee
WHERE employee_id=(SELECT employee_id 
                   FROM managers);

This sub query lists employees whom exist in both the employees and managers tables.

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