SQL Subquery

Summary: in this tutorial, you will learn about the SQL subquery and how to use the subqueries to form flexible SQL statements.

SQL subquery basic

Consider the following employees and departments tables from the sample database:

Suppose you have to find all employees who locate in the location with the id 1700. You might come up with the following solution.

First, find all departments located at the location whose id is 1700:

SELECT 
    *
FROM
    departments
WHERE
    location_id = 1700;

Second, find all employees that belong to location 1700 by using the department id list of the previous query:

SELECT 
    employee_id, first_name, last_name
FROM
    employees
WHERE
    department_id IN (1 , 3, 8, 10, 11)
ORDER BY first_name , last_name;

This solution has two problems. To start with, you have looked at the departments table to check which department belongs to the location in 1700. However, the original question was not referring to any specific departments; it referred to location 1700.

Because of the small data volume, you can get a list of departments easily. However, in a real system with high volume data, it might be problematic.

Another problem was that you have to revise the queries whenever you want to find employees locate in a different location.

A much better solution to this problem is to use a subquery. By definition, a subquery is a query nested inside another query such as SELECT, INSERT, UPDATE, or DELETE statement. In this tutorial, we are focusing on the subquery used with the SELECT statement.

In this example, you can rewrite combine the two queries above as follows:

SELECT 
    employee_id, first_name, last_name
FROM
    employees
WHERE
    department_id IN (SELECT 
            department_id
        FROM
            departments
        WHERE
            location_id = 1700)
ORDER BY first_name , last_name;

The query placed within the parentheses is called a subquery. It is also known as an inner query or inner select. The query that contains the subquery is called an outer query or an outer select.

To execute the query, first, the database system has to execute the subquery and substitute the subquery between the parentheses with its result – a number of department id located at the location 1700 – and then executes the outer query.

You can use a subquery in many places such as:

• With the IN or NOT IN operator
• With comparison operators
• With the EXISTS or NOT EXISTS operator
• With the ANY or ALL operator
• In the FROM clause
• In the SELECT clause

SQL subquery examples

Let’s take some examples of using the subqueries to understand how they work.

SQL subquery with the IN or NOT IN operator

In the previous example, you have seen how the subquery was used with the IN operator. The following example uses a subquery with the NOT IN operator to find all employees who do not locate at the location 1700:

SELECT 
    employee_id, first_name, last_name
FROM
    employees
WHERE
    department_id NOT IN (SELECT 
            department_id
        FROM
            departments
        WHERE
            location_id = 1700)
ORDER BY first_name , last_name;

SQL subquery with the comparison operator

The following syntax illustrates how a subquery is used with a comparison operator:

comparison_operator (subquery)

where the comparison operator is one of these operators:

• Equal (=)
• Greater than (>)
• Less than (<)
• Greater than or equal ( >=)
• Less than or equal (<=)
• Not equal ( !=) or (<>)

The following example finds the employees who have the highest salary:

SELECT 
    employee_id, first_name, last_name, salary
FROM
    employees
WHERE
    salary = (SELECT 
            MAX(salary)
        FROM
            employees)
ORDER BY first_name , last_name;

In this example, the subquery returns the highest salary of all employees and the outer query finds the employees whose salary is equal to the highest one.

The following statement finds all employees whose salaries are 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);    

In this example, first, the subquery returns the average salary of all employees. Then, the outer query uses the greater than the operator to find all employees whose salaries are greater than the average.

SQL subquery with the EXISTS or NOT EXISTS operator

The EXISTS operator checks for the existence of rows returned from the subquery. It returns true if the subquery contains any rows. Otherwise, it returns false.

The syntax of the EXISTS the operator is as follows:

EXISTS (subquery )

The NOT EXISTS the operator is opposite to the EXISTS operator.

NOT EXISTS (subquery)

The following example finds all departments which have at least one employee with a salary is greater than 10,000:

SELECT 
    department_name
FROM
    departments d
WHERE
    EXISTS( SELECT 
            1
        FROM
            employees e
        WHERE
            salary > 10000
                AND e.department_id = d.department_id)
ORDER BY department_name; 

Similarly, the following statement finds all departments that do not have any employee with a salary greater than 10,000:

SELECT 
    department_name
FROM
    departments d
WHERE
    NOT EXISTS( SELECT 
            1
        FROM
            employees e
        WHERE
            salary > 10000
                AND e.department_id = d.department_id)
ORDER BY department_name;  

SQL subquery with the ALL operator

The syntax of the subquery when it is used with the ALL the operator is as follows:

comparison_operator ALL (subquery)

The following condition evaluates to true if x is greater than every value returned by the subquery.

x > ALL (subquery)

For example, suppose the subquery returns three values one, two, and three. The following condition evaluates to true if x is greater than 3.

x > ALL (1,2,3)

The following query uses the GROUP BY clause and MIN() function to find the lowest salary by department:

SELECT 
    MIN(salary)
FROM
    employees
GROUP BY department_id
ORDER BY MIN(salary) DESC;

The following example finds all employees whose salaries are greater than the lowest salary of every department:

SELECT 
    employee_id, first_name, last_name, salary
FROM
    employees
WHERE
    salary >= ALL (SELECT 
            MIN(salary)
        FROM
            employees
        GROUP BY department_id)
ORDER BY first_name , last_name;

SQL subquery with the ANY operator

The following shows the syntax of a subquery with the ANY operator:

comparison_operator ANY (subquery)

For example, the following condition evaluates to true if x is greater than any value returned by the subquery. So the condition x > SOME (1,2,3) evaluates to true if x is greater than 1.

x > ANY (subquery)

Note that the SOME the operator is a synonym for the ANY operator so you can use them interchangeably.

The following query finds all employees whose salaries are greater than or equal to the highest salary in every department.

SELECT 
    employee_id, first_name, last_name, salary
FROM
    employees
WHERE
    salary >= SOME (SELECT 
            MAX(salary)
        FROM
            employees
        GROUP BY department_id);

In this example, the subquery finds the highest salary of employees in each department. The outer query looks at these values and determines which employees’ salaries are greater than or equal to any highest salary by the department.

SQL subquery in the FROM clause

You can use a subquery in the FROM clause of the SELECT a statement as follows:

SELECT 
    *
FROM
    (subquery) AS table_name

In this syntax, the table alias is mandatory because all tables in the FROM the clause must have a name.

Note that the subquery specified in the FROM a clause is called a derived table in MySQL or inline view in Oracle.

The following statement returns the average salary of every department:

SELECT 
    AVG(salary) average_salary
FROM
    employees
GROUP BY department_id;

You can use this query as a subquery in the FROM clause to calculate the average salary of departments as follows:

SELECT 
    ROUND(AVG(average_salary), 0)
FROM
    (SELECT 
        AVG(salary) average_salary
    FROM
        employees
    GROUP BY department_id) department_salary;

SQL Subquery in the SELECT clause

A subquery can be used anywhere an expression can be used in the SELECT clause. The following example finds the salaries of all employees, their average salary, and the difference between the salary of each employee and the average salary.

SELECT 
    employee_id,
    first_name,
    last_name,
    salary,
    (SELECT 
            ROUND(AVG(salary), 0)
        FROM
            employees) average_salary,
    salary - (SELECT 
            ROUND(AVG(salary), 0)
        FROM
            employees) difference
FROM
    employees
ORDER BY first_name , last_name;

Now you should understand what an SQL subquery is and how to use subqueries to form flexible SQL statements.