聚合不应出现在 WHERE 子句中，除非该聚合位于 HAVING 子句或选择列表所包含的子查询中，并且要对其进行聚合的列是外部引用。

An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.