SQL消息表查詢

Question

這是我的談話表：

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

，你可以看到每個會話可以包含2個或更多用戶。

我想獲得只有2個用戶在那裏的對話的id。 即只包含用戶的會話1 & 2，答案是會話1.

但我該如何得到它？

Answer 1

這將選擇具有用戶1或用戶2，或兩者的所有對話，但沒有其他人：

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

如果你也想，它們擁有完全用戶1和2的所有對話，並沒有其他人，你還必須添加和條件：

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 

如果用戶ID可以被複制，這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

你應該使用having子句。假設（的conversationId，用戶ID）是PK還是AK，您查詢的是：

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

編輯與1,2-用戶加入對話，這是不包括指數友好的方式按行相關子查詢和無功能。

 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

希望這可以幫助你，

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

sqlfiddle demo