Question

我有三个表 - 广告（Id，AgentId，AdDesc，AdTitle，EmailId，ADate），AdComment（ComId，电子邮件，评论，AdId）和MyLikes（LikeId，电子邮件，AdId）。在这里，我必须加入三个表格并根据每个广告ID（Ad.Id = AdComment.AdId）显示广告表的所有字段以及来自AdComment表的评论数量，以及基于每个广告ID的给定电子邮件来自MyLikes的喜欢的数量（Ad.Id = MyLikes.AdId）。

当我刚加入广告和广告评论时，我使用了

SELECT * , (
SELECT COUNT( c.AdId )
) AS ComCount
FROM Ad a
LEFT JOIN AdComment c ON a.Id = c.AdId 
GROUP BY a.Id
ORDER BY a.Id DESC

这很好。

当我加入Ad和MyLikes时，我使用了 -

SELECT a.Id, (
SELECT COUNT( * )
FROM MyLikes m
WHERE (
m.email = 'nik@gmail.com'
AND m.AdId = a.Id
)
) AS MyCount
FROM Ad a
LEFT JOIN MyLikes m ON a.Id = m.AdId
GROUP BY a.Id

这很好。

但是，当我尝试所有三个表连接时它不起作用，给评论计数提供了一些错误的值。为此，我用 -

SELECT a.Id,a.AgentId,a.EmailId,a.AdTitle,a.AdDesc, (
SELECT COUNT(m.AdId)
FROM MyLikes m
WHERE (
m.email = 'nik@gmail.com'
AND m.AdId = a.Id
)
) AS MyCount,(
SELECT COUNT( c.AdId )
) AS ComCount
FROM Ad a
LEFT JOIN MyLikes m ON a.Id = m.AdId LEFT JOIN AdComment c ON a.Id = c.AdId
GROUP BY a.Id ORDER BY a.Id DESC

如何加入这三个表并获得所需的结果。

Answer 1

假设我正确理解了您的问题，可以使用子查询来count代替joins中的数据：

select a.Id, a.AgentId, a.EmailId, a.AdTitle, a.AdDesc, 
       b.AdIdCount, c.AdIdCount
from Ad a
       left join (select AdId, count(*) AdIdCount 
                  from AdComment 
                  group by AdId) b ON a.Id = b.AdId 
       left join (select AdId, count(*) AdIdCount 
                  from MyLikes 
                  where email = 'nik@gmail.com'
                  group by AdId) c ON a.Id = c.AdId

如果您更喜欢correlated subqueries，那么这应该有效：

select a.Id, a.AgentId, a.EmailId, a.AdTitle, a.AdDesc, 
       (select count(*) 
        from AdComment b 
        where a.Id = b.AdId),
       (select count(*) 
        from MyLikes c 
        where c.email = 'nik@gmail.com' and 
            a.Id = c.AdId)
from Ad a

Answer 2

假设LikeID和ComId是MyLikes和AdComment表的唯一ID字段，那么您应该能够使用一对连接并计算每个表中的不同id字段： -

SELECT a.Id,
        a.AgentId,
        a.EmailId,
        a.AdTitle,
        a.AdDesc,
        COUNT(DISTINCT m.LikeId) AS MyCount,
        COUNT(DISTINCT c.ComId) AS ComCount
FROM Ad a
LEFT OUTER JOIN MyLikes m ON m.email = 'nik@gmail.com' AND a.Id = m.AdId
LEFT JOIN AdComment c ON a.Id = c.AdId

使用分组依据加入三个表来获取计数

2 个答案: