mysql - sql消息表查询

Question

这是我的对话表：

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

如您所见，每个对话可以包含 2 个或更多用户。

我正在尝试获取只有 2 个用户在那里的对话的 ID。即只包含用户 1 和 2 的对话，答案是对话 1。

但我怎么得到它？

Answer 1

你应该使用有子句。假设 ( conversationID, userID ) 是 PK 或 AK，您的查询是：

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

已编辑加入 1,2 个用户对话，这是一种索引友好的方法，没有关联子查询，也没有逐行功能。

      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 2

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)

如果您还想要所有只有用户 1 和 2 的对话，而没有其他人，您还必须添加一个 and 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 userID 可以重复，最好使用 distinct：

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 3

希望这对你有帮助，

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

sqlfiddle 演示