MySQL JOINED table 查询应该 return 零但不 return 那些行

Question

SELECT sg.date, sg.groupName, sg.highlights, 
        user.display_name as displayName,
        COUNT(uc.groupName) as cmtcnt
FROM `saved_groups` as sg
    LEFT JOIN `user` on user.email = sg.userName
    LEFT JOIN `user_comments` as uc on sg.groupName = uc.groupName
WHERE uc.deleted=0
GROUP BY sg.groupName

我有两个 table，saved_groups 和 user_comments，查询的第二行应该（确实有效）return 一个额外的列与每个组关联的评论数。

然而，当一个群组有零条与之关联的评论时，该群组就不会被 return 编辑。只有 return 行是那些有 > 0 条评论的行。期望的行为是对所有 groupNames 进行 returned，并为在 uc table.

中具有零个关联评论的那些行指定 0

我该如何修正这个查询？

我试过：IF(uc.deleted=1, 0, COUNT(uc.groupName)) as cmtcnt -- 但这没什么区别，returned 得到的结果相同。

此时，我不确定接下来要尝试什么。

建议？

更新：

试过这个：

SELECT sg.date, sg.groupName, sg.highlights, 
        user.display_name as displayName,
        COUNT(uc.groupName) as cmtcnt
FROM geom.saved_groups as sg
    JOIN geom.user on user.email = sg.userName
    JOIN geom.user_comments as uc on sg.groupName = uc.groupName
WHERE isnull(uc.deleted,0) in (0,1)
GROUP BY sg.groupName

得到： #1582 - Incorrect parameter count in the call to native function 'isnull'

Answer 1

检查是否 uc.deleted=0 并删除 where 条件。 the and 处理左连接的非空条件

SELECT sg.date, sg.groupName, sg.highlights, 
        user.display_name as displayName,
        COUNT(uc.groupName) as cmtcnt
FROM dbo.saved_groups as sg
    JOIN dbo.user on user.email = sg.userName
    LEFT JOIN dbo.user_comments as uc on sg.groupName = uc.groupName
    AND uc.deleted = 0
GROUP BY sg.groupName

Answer 2

您的 where 条件正在过滤记录并仅将那些在 uc 中有数据的记录添加到 return，因此左连接没有任何好处。你可以做 (uc.deleted=0 or uc.deleted is null) 要么将条件 uc.deleted 移动到 on 条件 with join

Answer 3

您想要所有记录吗？然后删除 WHERE 子句。您想要 0 用于没有计数的记录？使用 COALESCE。像这样：

SELECT sg.date
, sg.groupName
, sg.highlights
, user.display_name as displayName
, COALESCE(COUNT(uc.groupName), 0) as cmtcnt 
FROM `saved_groups` as sg 
LEFT JOIN `user` on user.email = sg.userName 
LEFT JOIN `user_comments` as uc on sg.groupName = uc.groupName 
AND uc.deleted = 0 -- to get only comments that have not been deleted, and not the deleted ones
GROUP BY sg.groupName

MySQL JOINED table 查询应该 return 零但不 return 那些行

MySQL JOINED table query should return zero but instead does not return those rows

mysql

sql

mariadb

更新：