查询SELECT语句的列数不同

Question

我得到以下代码：

使用的SELECT语句的列数不同。 我仍然从该站点搜索解决方案，但是在查询中找不到问题所在。 任何人都可以在这里为我提供帮助，为什么我得到= “使用的SELECT语句的列数不同” ，我该如何解决？

SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            S.created, 
            M.share_count, 
            U.username,
            U.last_login,
            M.uploads, 
            S.uid_fk AS 
            share_uid,
            S.ouid_fk AS share_ouid FROM 
            messages M, 
            users U, 
            friends F,
            message_share S 
            WHERE 
            F.friend_one='$uid' AND 
            U.uid = F.friend_one AND
            U.status='1' AND 
            F.friend_two != S.ouid_fk AND 
            M.uid_fk = S.ouid_fk AND F.role='fri' AND 
            S.msg_id_fk = M.msg_id group by msg_id)
            UNION
            (SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            M.share_count,
            U.username,
            U.last_login,
            M.uploads, '0' AS share_uid, '0' AS share_ouid 
            FROM messages M, users U, friends F WHERE F.friend_one='$uid' AND U.status='1' AND M.uid_fk=U.uid AND M.uid_fk = F.friend_two GROUP by msg_id ) ORDER BY created DESC

Answer 1

这是因为您正在执行UNION ，在这种情况下， SELECT两个部分都应包含完全相同的列数，这对您的方案不满意。 您的第一个SELECT部分包含10个选定的列，而第二个SELECT语句仅包含9个列，因此出现错误。

SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            S.created, 
            M.share_count, 
            U.username,
            U.last_login,
            M.uploads, 
            S.uid_fk AS 
            share_uid,
            S.ouid_fk AS share_ouid 
            FROM 
            messages M, 
            users U, 
            .........
            UNION
            (SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            NOW() as created //Add some default value
            M.share_count,
            U.username,
            U.last_login,
            M.uploads, 
            '0' AS share_uid, 
            '0' AS share_ouid 

Answer 2

两个SELECT语句必须包含相同的列数（第二个SELECT语句中缺少创建的列数）：

SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            S.created, 
            M.share_count, 
            U.username,
            U.last_login,
            M.uploads, 
            S.uid_fk AS 
            share_uid,
            S.ouid_fk AS share_ouid
 FROM messages M, users U, friends F, message_share S 
 WHERE F.friend_one='$uid'
 AND U.uid = F.friend_one
 AND U.status='1'
 AND F.friend_two != S.ouid_fk
 AND M.uid_fk = S.ouid_fk
 AND F.role='fri'
 AND S.msg_id_fk = M.msg_id
 GROUP BY msg_id

UNION

SELECT DISTINCT 
            M.msg_id, 
            M.uid_fk, 
            M.message,
            '0000-00-00' as created
            M.share_count,
            U.username,
            U.last_login,
            M.uploads,
            '0' AS share_uid,
            '0' AS share_ouid 
 FROM messages M, users U, friends F
 WHERE F.friend_one='$uid'
 AND U.status='1'
 AND M.uid_fk=U.uid
 AND M.uid_fk = F.friend_two
 GROUP BY msg_id

 ORDER BY created DESC