Question

我有一个user_to_room表

roomID | userID

我现在想检查，如果2个用户在同一个房间

SELECT roomID
FROM   room_to_user
WHERE  userID = 1 AND 2

如何通过JOIN实现这一目标？

Answer 1

自我加入：

SELECT DISTINCT u1.roomID AS roomID
FROM   user_to_room u1, user_to_room u2
WHERE  u1.userID = 1 AND u2.userID = 2
AND    u1.roomID = u2.roomID;

请参阅demo。

Answer 2

您还可以进行以下查询：

CREATE TABLE user_to_room (roomID INT, userID INT);

INSERT INTO user_to_room VALUES (101, 2);
INSERT INTO user_to_room VALUES (102, 3);
INSERT INTO user_to_room VALUES (102, 4);


    select distinct roomid 
from user_to_room a where exists 
(select top 1 1 from user_to_room where roomid = a.roomid and userid =3)
and userid = 4

检查demo此处

Answer 3

您可以使用的另一种解决方案是：

SELECT   roomID
FROM     user_to_room
WHERE    userID IN (2,3)
GROUP BY roomID
HAVING   COUNT(DISTINCT userID)=2

这将计算满足condiction userID IN (2,3)的每个房间的DISTINCT用户数。如果计数为2，则表示userID 2和userID 3都在同一个房间内。

检查2个用户是否与mySQL在同一个房间

3 个答案: