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 Products
GROUP BY Name
HAVING AVG( Quantity ) > 2* (
   SELECT AVG( Quantity )
   FROM SalesOrderItems 
 );

name	AVG( Products.Quantity )
Baseball Cap	62.000000
Shorts	80.000000
Tee Shirt	52.333333

The query executes as follows:

The subquery calculates the average quantity of items in the SalesOrderItems table.
The main query then goes through the Products table, calculating the average quantity per 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

This example finds 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 ProductID, LineID
FROM SalesOrderItems
GROUP BY ProductID, LineID
HAVING 2* AVG( Quantity ) > (
   SELECT Quantity
   FROM Products
   WHERE Products.ID = SalesOrderItems.ProductID );

ProductID	LineID
601	3
601	2
601	1
600	2
...	...

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 SalesOrderItems.ProductID.

A subquery with a comparison returns a single value

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