Duplicate column name on JOIN in mysql

Question

I've got a problem with this sql

SELECT COUNT( * )
FROM (

  SELECT *
  FROM `user` `t`
  JOIN `user_relation` r ON ( t.user_id = r.follower_id
  OR t.user_id = r.user_id )
  WHERE r.status = "active"
  AND (
    r.user_id =125
    OR r.follower_id =125
  )
  AND t.user_id !=125
  GROUP BY t.username
)sq

I always get an error: "#1060 - Duplicate column name 'user_id' " Can anyone help/explain, what I did wrong?

Thanks in advance Jan

Answer 1

You need to provide aliased column in inner query

SELECT COUNT( * )
FROM (

  SELECT t.*
  FROM `user` `t`
  JOIN `user_relation` r ON ( t.user_id = r.follower_id
  OR t.user_id = r.user_id )
  WHERE r.status = "active"
  AND (
    r.user_id =125
    OR r.follower_id =125
  )
  AND t.user_id !=125
  GROUP BY t.username
)sq

Since you are interested in count(*) only you can return either t.* or r.* or any one column , the condition being that column names in inner query MUST be unique or if are same in both tables than prefixed with table alias name.

Answer 2

In Inner Query

SELECT *
  FROM `user` `t`
  JOIN `user_relation` r ON ( t.user_id = r.follower_id
  OR t.user_id = r.user_id )
  WHERE r.status = "active"
  AND (
    r.user_id =125
    OR r.follower_id =125
  )
  AND t.user_id !=125
  GROUP BY t.username

In the **select * ** there are two columns t.user_id and r.user_id with same name.

You can't use *. You have to mention all column name.

Or

select t.* / select r.*