如何在WHERE子句（MySQL）中使用VIEW？

Question

我想在WHERE子句中使用视图的数据。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？

Answer 1

就像您将使用表一样：

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

[我警告您不要将not in与子查询一起使用。即使来自子查询的一个值是NULL，也不会返回任何行。因此，我建议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';

然后，DISTINCT只会增加开销，因此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';