MySQL 使用 group by 之后然后 IFNULL(COUNT(*),0) 为什么还是会获得 null

Question

如题

MySQL 使用 group by 之后然后 IFNULL(COUNT(*),0) 为什么还是会获得 null。

 SELECT IFNULL(count(max_choose_number),0)   FROM t_paper_**** tps WHERE tps.paper_id = 210 AND tps.structure_type = 4 GROUP BY tps.paper_id

结果为 null

Answer 1

In this case, there is generally no data that satisfies the where condition. Therefore, null is returned.