SQL消息表查询

Question

这是我的谈话表：

conversationID || userID 
     1   || 1 
     1   || 2 
     2   || 1 
     2   || 2 
     2   || 3 

，你可以看到每个会话可以包含2个或更多用户。

我想获得只有2个用户在那里的对话的id。 即只包含用户的会话1 & 2，答案是会话1.

但我该如何得到它？

Answer 1

这将选择具有用户1或用户2，或两者的所有对话，但没有其他人：

select conversationID 
from conversations 
group by conversationID 
having count(*) = count(case when userID in (1,2) then 1 end) 

如果你也想，它们拥有完全用户1和2的所有对话，并没有其他人，你还必须添加和条件：

select conversationID 
from conversations 
group by conversationID 
having count(*) = count(case when userID in (1,2) then 1 end) 
     and count(*) = 2 -- number of elements in set 

如果用户ID可以被复制，这也是更好地使用不同的：

select conversationID 
from conversations 
group by conversationID 
having 
    count(distinct userID) = count(distinct case when userID in (1,2) then userID end) 
    and count(distinct userID) = 2 -- number of elements in set 

Answer 2

你应该使用having子句。假设（的conversationId，用户ID）是PK还是AK，您查询的是：

select conversationID 
    from your_Table 
group by conversationID 
    having count(*) = 2 

编辑与1,2-用户加入对话，这是不包括指数友好的方式按行相关子查询和无功能。

 select t1 conversationID 
     from your_Table t1 
    inner join 
       (select distinct conversationID 
        from your_Table 
       where userId in (1, 2) 
       ) t2 
      on t1.conversationID = t2.conversationID 
    group by t1.conversationID 
     having count(distinct t1.userId) = 2 

Answer 3

希望这可以帮助你，

select conversationID from conversation 
group by ConversationID having count(distinct UserID)=2; 

sqlfiddle demo