MySQL Subquery

 MySQL Subquery

Summary: in this tutorial, we will show you how to use the MySQL subquery to write complex queries and explain the correlated subquery concept.

A MySQL subquery is a query nested within another query such as SELECT, INSERT, UPDATE or DELETE. In addition, a subquery can be nested inside another subquery.

A MySQL subquery is called an inner query while the query that contains the subquery is called an outer query. A subquery can be used anywhere that expression is used and must be closed in parentheses.

The following query returns employees who work in offices located in the USA.

SELECT 
    lastName, firstName
FROM
    employees
WHERE
    officeCode IN (SELECT 
            officeCode
        FROM
            offices
        WHERE
            country = 'USA');

In this example:

• The subquery returns all office codes of the offices located in the USA.
• The outer query selects the last name and first name of employees who work in the offices whose office codes are in the result set returned by the subquery.

When the query is executed, the subquery runs first and returns a result set. Then, this result set is used as an input for the outer query.

MySQL subquery in WHERE clause

We will use the table payments in the sample database for the demonstration.

MySQL subquery with comparison operators

You can use comparison operators e.g., =, >, < to compare a single value returned by the subquery with the expression in the WHERE clause.

For example, the following query returns the customer who has the maximum payment.

SELECT 
    customerNumber, 
    checkNumber, 
    amount
FROM
    payments
WHERE
    amount = (SELECT MAX(amount) FROM payments);

In addition to the equality operator, you can use other comparison operators such as greater than ( >), less than( <).

For example, you can find customers whose payments are greater than the average payment using a subquery:

SELECT 
    customerNumber, 
    checkNumber, 
    amount
FROM
    payments
WHERE
    amount > (SELECT 
            AVG(amount)
        FROM
            payments);

In this example:

• First, use a subquery to calculate the average payment using the AVG aggregate function.
• Then, query the payments that are greater than the average payment returned by the subquery in the outer query.

MySQL subquery with IN and NOT IN operators

If a subquery returns more than one value, you can use other operators such as IN or NOT IN operator in the WHERE clause.

See the following customers and orders tables:

For example, you can use a subquery with NOT IN operator to find the customers who have not placed any orders as follows:

SELECT 
    customerName
FROM
    customers
WHERE
    customerNumber NOT IN (SELECT DISTINCT
            customerNumber
        FROM
            orders);

MySQL subquery in the FROM clause

When you use a subquery in the FROM clause, the result set returned from a subquery is used as a temporary table. This table is referred to as a derived table or materialized subquery.

The following subquery finds the maximum, minimum, and average number of items in sale orders:

SELECT 
    MAX(items), 
    MIN(items), 
    FLOOR(AVG(items))
FROM
    (SELECT 
        orderNumber, COUNT(orderNumber) AS items
    FROM
        orderdetails
    GROUP BY orderNumber) AS lineitems;

Note that the FLOOR() is used to remove decimal places from the average values of items.

MySQL correlated subquery

In the previous examples, you notice that a subquery is independent. It means that you can execute the subquery as a standalone query, for example:

SELECT 
    orderNumber, 
    COUNT(orderNumber) AS items
FROM
    orderdetails
GROUP BY orderNumber;

Unlike a standalone subquery, a correlated subquery is a subquery that uses the data from the outer query. In other words, a correlated subquery depends on the outer query. A correlated subquery is evaluated once for each row in the outer query.

In the following query, we select products whose buy prices are greater than the average buy price of all products in each product line.

SELECT 
    productname, 
    buyprice
FROM
    products p1
WHERE
    buyprice > (SELECT 
            AVG(buyprice)
        FROM
            products
        WHERE
            productline = p1.productline)

The inner query executes for every product line because the product line is changed for every row. Hence, the average buy price will also change. The outer query filters only products whose buy price is greater than the average buy price per product line from the subquery.

MySQL subquery with EXISTS and NOT EXISTS

When a subquery is used with the EXISTS or NOT EXISTS operator, a subquery returns a Boolean value of TRUE or FALSE.  The following query illustrates a subquery used with the EXISTS operator:

SELECT 
    *
FROM
    table_name
WHERE
    EXISTS( subquery );

In the query above, if the subquery returns any rows, EXISTS subquery returns TRUE, otherwise, it returns FALSE.

The EXISTS and NOT EXISTS are often used in the correlated subqueries.

Let’s take a look at the orders and orderdetails tables from the sample database:

The following query finds sales orders whose total values are greater than 60K.

SELECT 
    orderNumber, 
    SUM(priceEach * quantityOrdered) total
FROM
    orderdetails
        INNER JOIN
    orders USING (orderNumber)
GROUP BY orderNumber
HAVING SUM(priceEach * quantityOrdered) > 60000;

It returns 3 rows, meaning that there are 3 sales orders whose total values are greater than 60K.

You can use the query above as a correlated subquery to find customers who placed at least one sales order with a total value greater than 60K by using the EXISTS operator:

SELECT 
    customerNumber, 
    customerName
FROM
    customers
WHERE
    EXISTS( SELECT 
            orderNumber, SUM(priceEach * quantityOrdered)
        FROM
            orderdetails
                INNER JOIN
            orders USING (orderNumber)
        WHERE
            customerNumber = customers.customerNumber
        GROUP BY orderNumber
        HAVING SUM(priceEach * quantityOrdered) > 60000);

In this tutorial, we have shown you how to use MySQL subquery and correlated subquery to construct more complex queries.