In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. Because the subquery may be evaluated once for each row processed by the outer query, it can be inefficient.
Here is an example for a typical correlated subquery. In this example, the objective is to find all employees whose salary is above average for their department.
 SELECT employee_number, name
   FROM employees AS emp
   WHERE salary > (
     SELECT AVG(salary)
       FROM employees
       WHERE department = emp.department);
In the above query the outer query is
 SELECT employee_number, name
   FROM employees AS emp
   WHERE salary > ...
and the inner query (the correlated subquery) is
 SELECT AVG(salary)
   FROM employees
   WHERE department = emp.department
In the above nested query the inner query has to be re-executed for each employee. (A sufficiently smart implementation may cache the inner query's result on a department-by-department basis, but even in the best case the inner query must be executed once per department. See "Optimizing correlated subqueries" below.)
Correlated subqueries may appear elsewhere besides the WHERE clause; for example, this query uses a correlated subquery in the SELECT clause to print the entire list of employees alongside the average salary for each employee's department. Again, because the subquery is correlated with a column of the outer query, it must be re-executed for each row of the result.
 SELECT
   employee_number,
   name,
   (SELECT AVG(salary) 
      FROM employees
      WHERE department = emp.department) AS department_average
   FROM employees AS emp;