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

Understanding the Limitations of Using Aliases in HAVING Clauses

In SQL, it might seem intuitive to utilize aliases for aggregates within the HAVING clause. However, this approach results in the error "Invalid column name" due to the specific evaluation order of SQL queries.

To comprehend this limitation, it is crucial to understand how SQL executes a query:

1. It first creates a result set by combining all rows from the joined tables specified in the FROM clause.
2. The WHERE clause filters out rows that do not meet its criteria.
3. It then groups rows based on the columns specified in the GROUP BY clause.
4. Critically, the HAVING clause is evaluated before the SELECT statement. This means that SQL does not yet recognize aliases defined in the SELECT clause at this point.
5. Only after evaluating the HAVING clause does SQL proceed to evaluate the expressions in the SELECT list.

Therefore, since the HAVING clause is evaluated prior to the SELECT clause, the alias assigned to the aggregate in the SELECT statement (e.g., "col7") is not yet known to SQL when it evaluates the HAVING expression. Consequently, the error "Invalid column name 'col7'" occurs.

In contrast, the ORDER BY clause is evaluated after the SELECT statement, allowing aliases defined in the SELECT clause to be used. This is because the ORDER BY clause operates on the result set created by the SELECT statement.

To remedy this issue, replace the alias in the HAVING clause with the actual aggregate expression, as shown below:

select col1,count(col2)
from --some join operation
group by col1
having count(col2) >= 3

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