MySQL：按选择条件更新

Question

我有问答系统， 每个问题应该有4个答案

如果问题的答案少于4个，我需要运行SQL查询将问题的“类”更新为“D”（以防止它出现）

表格结构：

问题（表）

id     text     class
1      Is..      A
2      Where..   B
3      where..   A
4      Why..     A 
5      Do..      A

回答（表）

id     qid     text
1       1       ..
2       1       ..
3       1       ..
4       2       ..
5       3       ..

注意：“answer.qid”指的是“question.id”

感谢，

Answer 1

尝试使用子查询，例如：

UPDATE question
SET class = 'D'
WHERE id IN 
         (SELECT qid
          FROM answer
          GROUP BY qid
          HAVING count(qid) < 4)

Answer 2

我想说，您可以使用子选择：

UPDATE question SET class='D' WHERE ( SELECT count(*) FROM answer INNER JOIN answer.qid = question.id ) < 4;

Answer 3

update question
 set class='D'
 where question.id in (select qid from answer group by qid having count(*)<4)