An Essential Guide to MySQL Derived Table

 An Essential Guide to MySQL Derived Table

Summary: in this tutorial, you will learn about the MySQL-derived table and how to use it to simplify complex queries.

Introduction to MySQL-derived table

A derived table is a virtual table returned from a SELECT statement. A derived table is similar to a temporary table, but using a derived table in the SELECT statement is much simpler than a temporary table because it does not require steps of creating the temporary table.

The term derived table and subquery is often used interchangeably. When a stand-alone subquery is used in the FROM clause of a SELECT the statement, it is called a derived table.

The following illustrates a query that uses a derived table:

Note that a stand-alone subquery is a subquery that can execute independently of the statement containing it.

Unlike a subquery, a derived table must have an alias so that you can reference its name later in the query. If a derived table does not have an alias, MySQL will issue the following error:

Every derived table must have its own alias.

The following illustrates an SQL statement that uses a derived table:

SELECT 
    column_list
FROM
    (SELECT 
        column_list
    FROM
        table_1) derived_table_name;
WHERE derived_table_name.c1 > 0;

A simple MySQL derived table example

The following query gets the top 5 products by sales revenue in 2003 from the orders and orderdetails tables in the sample database:

SELECT 
    productCode, 
    ROUND(SUM(quantityOrdered * priceEach)) sales
FROM
    orderdetails
        INNER JOIN
    orders USING (orderNumber)
WHERE
    YEAR(shippedDate) = 2003
GROUP BY productCode
ORDER BY sales DESC
LIMIT 5;

You can use the result of this query as a derived table and join it with the products table as follows:

SELECT 
    productName, sales
FROM
    (SELECT 
        productCode, 
        ROUND(SUM(quantityOrdered * priceEach)) sales
    FROM
        orderdetails
    INNER JOIN orders USING (orderNumber)
    WHERE
        YEAR(shippedDate) = 2003
    GROUP BY productCode
    ORDER BY sales DESC
    LIMIT 5) top5products2003
INNER JOIN
    products USING (productCode);

The following shows the output of the query above:

In this example:

1. First, the subquery is executed to create a result set or derived table.
2. Then, the outer query is executed that joined the top5product2003 derived table with the products table using the productCode column.

A more complex MySQL derived table example

Suppose you have to classify the customers  who bought products in 2003 into 3 groups: platinum, gold, and silver. And you need to know the number of customers in each group with the following conditions:

1. Platinum customers who have orders with a volume greater than 100K
2. Gold customers who have orders with a volume between 10K and 100K
3. Silver customers who have orders with a volume of less than 10K

To construct this query, you first need to put each customer into the respective group using CASE expression and GROUP BY clause as follows:

SELECT 
    customerNumber,
    ROUND(SUM(quantityOrdered * priceEach)) sales,
    (CASE
        WHEN SUM(quantityOrdered * priceEach) < 10000 THEN 'Silver'
        WHEN SUM(quantityOrdered * priceEach) BETWEEN 10000 AND 100000 THEN 'Gold'
        WHEN SUM(quantityOrdered * priceEach) > 100000 THEN 'Platinum'
    END) customerGroup
FROM
    orderdetails
        INNER JOIN
    orders USING (orderNumber)
WHERE
    YEAR(shippedDate) = 2003
GROUP BY customerNumber;

The following is the output of the query:

Then, you can use this query as the derived table and perform grouping as follows:

SELECT 
    customerGroup, 
    COUNT(cg.customerGroup) AS groupCount
FROM
    (SELECT 
        customerNumber,
            ROUND(SUM(quantityOrdered * priceEach)) sales,
            (CASE
                WHEN SUM(quantityOrdered * priceEach) < 10000 THEN 'Silver'
                WHEN SUM(quantityOrdered * priceEach) BETWEEN 10000 AND 100000 THEN 'Gold'
                WHEN SUM(quantityOrdered * priceEach) > 100000 THEN 'Platinum'
            END) customerGroup
    FROM
        orderdetails
    INNER JOIN orders USING (orderNumber)
    WHERE
        YEAR(shippedDate) = 2003
    GROUP BY customerNumber) cg
GROUP BY cg.customerGroup;    

The query returns the customer groups and the number of customers in each.

In this tutorial, you have learned how to use the MySQL derived tables which are subqueries in the FROM clause to simplify complex queries.