在mysql中多次联接一个表

Question

I've two tables for user and group member info. I am storing two user ids in groupmember table one as group_userid and another one as group_memberid . I want to join user table with group_userid and with group_memberi d. I've tried this query:

SELECT U.*, 
  S1.user_id as folusrid,
  S1.first_name,
  S1.last_name, 
  S2.user_id as follwginuserid,
  S2.first_name,
  S2.last_name 
FROM tbl_groupmemebers U 
JOIN tbl_user S1 ON (S1.user_id = U.group_userid) 
JOIN tbl_user S2 ON (S2.user_id = U.group_memberid) 
WHERE U.group_id = '2'

and got the result but the result is returning me in same row like this:

id    group_id    group_userid    group_memberid    group_joindate        folusrid    first_name        last_name    follwginuserid    first_name    last_name
3        2            18                11          2015-04-24 12:02:36     18        Mohammadshoaib    Swichmann           11           Clemen      Swichmann

I want the result in two separate columns.

Answer 1

Try this UNION

(
  SELECT U.*, 
    S1.user_id as user_id,
    S1.first_name,
    S1.last_name,
    'group_userid' as data_type
  FROM tbl_groupmemebers U 
  JOIN tbl_user S1 ON (S1.user_id = U.group_userid) 
  WHERE U.group_id = '2'
)
UNION ALL
(
  SELECT U.*, 
    S2.user_id as user_id,
    S2.first_name,
    S2.last_name,
    'group_userid' as data_type
  FROM tbl_groupmemebers U
  JOIN tbl_user S2 ON (S2.user_id = U.group_memberid) 
  WHERE U.group_id = '2'
);

This will give you one record for every U.group_userid and one record for every U.group_memberid. Use the 'data_type' column (the last one) to identify which one is what.