About Correlated Subqueries

Correlated subqueries are subqueries ( sub query is a query nested inside another query) where the inner query has a reference to a column from the table in the outer query. you can’t just highlight the inner portion and run it independently. For example

SELECT productid, productname, unitprice FROM Production.Products WHERE unitprice in (SELECt sale_price from sale_product where Production.Products=Sale_Products.productid ) FROM Production.Products);

Here is another 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.

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;