A query is an operation that retrieves data from one or more tables or views. In this reference, a top-level SELECT statement is called a query, and a query nested within another SQL statement is called a subquery
A subquery answers multiple-part questions. For example, to determine who works in Blake’s department, you can first use a subquery to determine the Blake’s employee number. You can then answer the original question with the parent SELECT statement. A subquery in the FROM clause of a SELECT statement is also called an inline view. A subquery in the WHERE clause of a SELECT statement is also called a nested subquery.
Example
SELECT FIRST_NAME, LAST_NAME, EMPLOYEE_ID FROM EMPLOYEES WHERE MANAGER_ID = (SELECT MANAGER_ID FROM EMPLOYEES WHERE LAST_NAME = 'Greenberg')