Correlated subquery
Correlated subquery
A correlated subquery (or synchronized subquery) is a subquery that uses values from the outer query.Because the subquery is evaluated once for each row processed by the outer query, it can be inefficient.
Example
The list of all employees whose salary is above average for their departments:SELECT employee_number, name
FROM employees AS Bob
WHERE salary > (
SELECT AVG(salary)
FROM employees
WHERE department = Bob.department);
SELECT employee_number, name
FROM employees AS Bob
WHERE salary > ...
SELECT AVG(salary)
FROM employees
WHERE department = Bob.department
In the above nested query the inner query has to be re-executed for each employee.
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