Unknown column error in this COUNT MySQL statement?

Question

Error is

Unknown column \'num\' in \'where\' clause

SELECT COUNT(*) AS num, books_bookid
FROM bookgenre_has_books
WHERE         


        

Answer 1

WHERE clause cant see aliases,use HAVING.

It is not allowable to refer to a column alias in a WHERE clause, because the column value might not yet be determined when the WHERE clause is executed

http://dev.mysql.com/doc/refman/5.0/en/problems-with-alias.html