SELECT clause is logically processed after the
HAVING clause. Therefore the aliases used in
SELECT don't exist (yet) when the
HAVING clause is processed.
On MSDN you can look at SELECT (Transact-SQL):
Logical Processing Order of the SELECT statement
The following steps show the logical processing order, or binding order, for a SELECT statement. This order determines when the objects defined in one step are made available to the clauses in subsequent steps. For example, if the query processor can bind to (access) the tables or views defined in the FROM clause, these objects and their columns are made available to all subsequent steps. Conversely, because the SELECT clause is step 8, any column aliases or derived columns defined in that clause cannot be referenced by preceding clauses. However, they can be referenced by subsequent clauses such as the ORDER BY clause. Note that the actual physical execution of the statement is determined by the query processor and the order may vary from this list.
- GROUP BY
- WITH CUBE or WITH ROLLUP
- ORDER BY
This query works because it repeats what will later be defined (the
COUNT) in the
SELECT OrderID, COUNT(ProductID) products
GROUP BY OrderID
HAVING COUNT(ProductID) > 5;