Sub Queries

Sub queries is select statement that appears as part of another SQL statement.  Sub query is used when comparing data from different tables is parenthesized whiting the where clause.

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

The above sample lists first name and last name of all the employees from section 1112.  You can tell that the section ID is not in the employee table department ID is, and section ID is in the department table.

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

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