Question

所以在facebook这样的墙上我们有实体：海报。（id，user_id，message）用户。（id，name）连接。（user1_id，user2_id）

我想用一个SQL查询来获取id为x的用户的所有墙帖（示例5）我试过了：

select wallPost.* from 
wallPost, Connections
Where 
wallPost.user_id = x
or 
(Connections.user1_id =x and wallPost.user_id = connections.user2_id) 
or  
(Connections.user2_id =x and wallPost.user_id = connections.user1_id) 
ORDER BY <field_name>

除了可能产生的性能问题之外，当用户没有任何连接时，它也无法正常工作。我不想使用两个查询，因为我不想对结果进行排序。

谢谢

Answer 1

您只需要wallPost的记录。我建议您删除Connections子句中的from表，并使用exists将逻辑放在子查询中：

select wp.*
from wallPost wp 
where wp.user_id = x or 
      exists (select 1 from Connections c where c.user1_id = x and wp.user_id = c.user2_id) or
      exists (select 1 from Connections c where c.user2_id = x and wp.user_id = c.user1_id)
order by <field_name>;

为了获得最佳效果，您需要两个索引，一个在Connections(user2_id, user1_id)上，另一个在Connections(user1_id, user2_id)上。

Answer 2

select wp.*
from wallPost wp 
where 
wp.user_id = x 
or 
(wp.user_id in
      (select c.user2_id from connections c where c.user1_id = x ) 
)
or
(wp.user_id in (
      (select c.user1_id from connections c where c.user2_id = x )))

Answer 3

为什么你不能这样做？

    SELECT WP.* FROM [WallPost] WP WITH (NOLOCK)  
       LEFT JOIN [Connections] CN WITH (NOLOCK) 
          ON ((WP.[User_Id] = CN.[User1_Id]) OR (WP.[User_Id] = CN.[User2_Id))
    WHERE (WP.[User_Id] = X)

LEFT JOIN将照顾没有联系的用户。

有没有办法用3个实体改进这个SQL查询？

3 个答案: