如何在 WHERE 子句（MySQL）中使用 VIEW？

Posted 2023-02-26

【中文标题】如何在 WHERE 子句（MySQL）中使用 VIEW？

【英文标题】：How to use VIEW in WHERE clause (MySQL)?

【发布时间】：2020-03-28 11:41:05

【问题描述】：

我想在 WHERE 子句中使用视图数据。但是出现错误：

create view post_with_answers AS
    SELECT DISTINCT postid 
    FROM (SELECT postid FROM `qa_posts` WHERE `type` = 'Q') AS q1
    INNER JOIN (SELECT parentid FROM `qa_posts` WHERE `type` = 'A') AS q2 ON q1.postid = q2.parentid

select count(*)
from qa_posts
where parentid not in post_with_answers

在最后一行我收到此错误： SQL Error [1064] [42000]: You have an error in your SQL syntax; check the manual that corresponds to your mysql server version for the right syntax to use near 'post_with_answers' at line 3

如何解决？

【参考方案1】：

就像使用表格一样：

select count(*)
from qa_posts
where parentid not in (select pwa.postid from post_with_answers pwa);

我会提醒您不要将not in 与子查询一起使用。如果子查询中的一个值是NULL，则不会返回任何行。为此，我推荐NOT EXISTS：

select count(*)
from qa_posts p
where not exists (select 1
                  from post_with_answers pwa
                  where p.parentid = pwa.postid
                 );

此外，您的视图定义有点过于复杂。您不需要子查询：

create view post_with_answers AS
    SELECT DISTINCT pq.postid 
    FROM qa_posts pq JOIN
         qa_posts pa
         ON pq.postid = pa.parentid
    WHERE pq.type = 'Q' AND pa.type = 'A';

那么，DISTINCT 只是增加了开销，所以EXISTS 更好：

create view post_with_answers AS
    SELECT DISTINCT pq.postid 
    FROM qa_posts pq 
    WHERE EXISTS (SELECT 1
                  FROM qa_posts pa
                  WHERE pq.postid = pa.parentid AND
                        pa.type = 'A'
                 )
    WHERE pq.type = 'Q';