What is a derived table when is it used Can you describe any situations WHERE you would have to use it over a subquery in the WHERE clause?

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

Introduction to MySQL derived tables

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 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 statement, it is also 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 outer query.

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 the syntax of a query that uses a derived table:

SELECT select_list FROM (SELECT select_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 five 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:

• Platinum customers who have orders with the volume greater than 100K.
• Gold customers who have orders with the volume between 10K and 100K.
• Silver customers who have orders with the volume less than 10K.

To form 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.

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