SQL Correlated Subquery

Summary: in this tutorial, you will learn about the SQL correlated subquery which is a subquery that uses values from the outer query.

Introduction to SQL correlated subquery #

Let’s start with an example.

See the following employees table in the sample database:

The following query finds employees whose salary is greater than the average salary of all employees:

SELECT
  employee_id,
  first_name,
  last_name,
  salary
FROM
  employees
WHERE
  salary > (
    SELECT
      AVG(salary)
    FROM
      employees
  );Code language: SQL (Structured Query Language) (sql)

Try it

In this example, the subquery is used in the WHERE clause. There are some points that you can see from this query:

First, you can execute the subquery that returns the average salary of all employees independently.

SELECT
  AVG(salary)
FROM
  employees;Code language: SQL (Structured Query Language) (sql)

Try it

Second, the database system needs to evaluate the subquery only once.

Third, the outer query makes use of the result returned from the subquery. The outer query depends on the subquery for its value. However, the subquery does not depend on the outer query. Sometimes, we call this subquery is a plain subquery.

Unlike a plain subquery, a correlated subquery is a subquery that uses the values from the outer query. Also, a correlated subquery may be evaluated once for each row selected by the outer query. Because of this, a query that uses a correlated subquery may be slow.

A correlated subquery is also known as a repeating subquery or a synchronized subquery.

SQL correlated subquery examples #

Let’s see few more examples of the correlated subqueries to understand them better.

SQL correlated subquery in the WHERE clause example #

The following query finds all employees whose salary is higher than the average salary of the employees in their departments:

SELECT
  employee_id,
  first_name,
  last_name,
  salary,
  department_id
FROM
  employees e
WHERE
  salary > (
    SELECT
      AVG(salary)
    FROM
      employees
    WHERE
      department_id = e.department_id
  )
ORDER BY
  department_id,
  first_name,
  last_name;Code language: SQL (Structured Query Language) (sql)

Try it

Here is the output:

In this example, the outer query is:

SELECT 
    employee_id, 
    first_name, 
    last_name, 
    salary, 
    department_id
FROM
    employees e
WHERE
    salary >
...
Code language: SQL (Structured Query Language) (sql)

and the correlated subquery is:

SELECT
    AVG( list_price )
FROM
    products
WHERE
    category_id = p.category_id
Code language: SQL (Structured Query Language) (sql)

For each employee, the database system has to execute the correlated subquery once to calculate the average salary of the employees in the department of the current employee.

SQL correlated subquery in the SELECT clause example #

The following query returns the employees and the average salary of all employees in their departments:

SELECT
  employee_id,
  first_name,
  last_name,
  department_name,
  salary,
  (
    SELECT
      ROUND(AVG(salary), 0)
    FROM
      employees
    WHERE
      department_id = e.department_id
  ) avg_salary_in_department
FROM
  employees e
  INNER JOIN departments d ON d.department_id = e.department_id
ORDER BY
  department_name,
  first_name,
  last_name;Code language: SQL (Structured Query Language) (sql)

Try it

Output:

For each employee, the database system has to execute the correlated subquery once to calculate the average salary by the employee’s department.

SQL correlated subquery with EXISTS operator example #

We often use a correlated subquery with the EXISTS operator. For example, the following query returns all employees who have no dependents:

SELECT
  employee_id,
  first_name,
  last_name
FROM
  employees e
WHERE
  NOT EXISTS (
    SELECT
      *
    FROM
      dependents d
    WHERE
      d.employee_id = e.employee_id
  )
ORDER BY
  first_name,
  last_name;Code language: SQL (Structured Query Language) (sql)

Try it

The following picture shows the output:

In this tutorial, you have learned about the SQL correlated subquery and how to apply it to form a complex query.