With the use of HAVING you can use a result columns of a database query in itself:
SELECT category_field = 3 AS 'result' FROM catalog_category_entity WHERE condition_field = 985 HAVING result = 1;
Here the alias result (see AS ‘result’) for the only column is used as a condition with HAVING. That is by now the only way to compare a result column with another value in a query under MySQL.
WHERE result = 1;
Would not work!!!
Attention: HAVING is slower than a normal WHERE clause!