Why can’t I use an alias for an aggregate in a having clause?

The HAVING clause is evaluated before the SELECT – so the server doesn’t yet know about that alias.

1. First, the product of all tables in the FROM clause is formed.

2. The WHERE clause is then evaluated to eliminate rows that do not satisfy the search_condition.

3. Next, the rows are grouped using the columns in the GROUP BY clause.

4. Then, groups that do not satisfy the search_condition in the HAVING clause are eliminated.

5. Next, the expressions in the SELECT statement target list are evaluated.

6. If the DISTINCT keyword in present in the select clause, duplicate rows are now eliminated.

7. The UNION is taken after each sub-select is evaluated.

8. Finally, the resulting rows are sorted according to the columns specified in the ORDER BY clause.

9. TOP clause is executed.

Hope this answers your question. Also, it explains why the alias works in ORDER BY clause.