You can't use aliases in a WHERE clause. As this page explains:
Standard SQL doesn't allow you to refer to a column alias in a WHERE clause. This restriction is imposed because when the WHERE code is executed, the column value may not yet be determined. For example, the following query is illegal: SELECT id, COUNT(*) AS cnt FROM tbl_name WHERE cnt > 0 GROUP BY id; The WHERE statement is executed to determine which rows should be included in the GROUP BY part, whereas HAVING is used to decide which rows from the result set should be used.