Subqueries in the HAVING clause

Subqueries in the HAVING clause

Although you usually use subqueries as search conditions in the WHERE clause, sometimes you can also use them in the HAVING clause of a query. When a subquery appears in the HAVING clause, like any expression in the HAVING clause, it is used as part of the row group selection.

Here is a request that lends itself naturally to a query with a subquery in the HAVING clause: "Which products' average in-stock quantity is more than double the average number of each item ordered per customer?"

Example

SELECT name, avg(quantity)
FROM product
GROUP BY name
HAVING avg(quantity) > 2* (
   SELECT avg(quantity)
   FROM sales_order_items
   )

name	avg(product.quantity)
Tee Shirt	52.333333
Baseball Cap	62
Shorts	80

The query executes as follows:

The subquery calculates the average quantity of items in the sales_order_items table.
The main query then goes through the product table, calculating the average quantity product, grouping by product name.
The HAVING clause then checks if each average quantity is more than double the quantity found by the subquery. If so, the main query returns that row group; otherwise, it doesn't.
The SELECT clause produces one summary row for each group, displaying the name of each product and its in-stock average quantity.

You can also use outer references in a HAVING clause, as shown in the following example, a slight variation on the one above.

Example

"Find the product ID numbers and line ID numbers of those products whose average ordered quantities is more than half the in-stock quantities of those products."

SELECT prod_id, line_id
FROM sales_order_items
GROUP BY prod_id, line_id
HAVING 2* avg(quantity) > (
   SELECT quantity
   FROM product
   WHERE product.id = sales_order_items.prod_id)

prod_id	line_id
401	2
401	1
401	4
501	3
...	...

In this example, the subquery must produce the in-stock quantity of the product corresponding to the row group being tested by the HAVING clause. The subquery selects records for that particular product, using the outer reference sales_order_items.prod_id.

A subquery with a comparison returns a single value

This query uses the comparison ">", suggesting that the subquery must return exactly one value. In this case, it does. Since the id field of the product table is a primary key, there is only one record in the product table corresponding to any particular product id.

Subquery tests