Why Can't I Use Aliased Aggregates in SQL's HAVING Clause?

Why Aliased Aggregates Are Prohibited in HAVING Clauses

In SQL, users often encounter an error when attempting to use an alias for an aggregate in a HAVING clause. This stems from the specific order in which SQL processes queries.

The HAVING clause, which filters the results of a GROUP BY operation, is evaluated before the SELECT clause. As a result, aliases defined in the SELECT clause are not yet available when evaluating the HAVING clause.

To understand this better, let's consider the following logical sequence of query execution:

1. The results of all joined tables in the FROM clause are formed.
2. The WHERE clause filters out rows that do not meet the search criteria.
3. Rows are grouped according to the GROUP BY clause.
4. Groups that do not meet the HAVING clause criteria are eliminated.
5. Aggregate functions are applied to calculate aggregate values.
6. Expressions in the SELECT clause are evaluated.

Since the HAVING clause is evaluated before the SELECT clause, it cannot reference aliases defined later in the query. This restriction explains why using an alias in a HAVING clause, as shown in the example, results in an "Invalid column name" error.

In contrast, aliases work in the ORDER BY clause because the ORDER BY clause is evaluated after the SELECT clause. Therefore, aliases defined in the SELECT clause are available for use in the ORDER BY clause.

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