Why can’t I use alias in a count(*) “column” and reference it in a having clause?

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See the document referenced by CodeByMoonlight in an answer to your recent question.

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 clause 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.

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