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create table t (id int, user_id int, req_user_id int);
insert into t (id, user_id, req_user_id) values
(1, 1, 2),
(2, 2, 9),
(3, 7, 2);
Records: 3 Duplicates: 0 Warnings: 0
select * from t
| id | user_id | req_user_id |
|---|---|---|
| 1 | 1 | 2 |
| 2 | 2 | 9 |
| 3 | 7 | 2 |
select group_concat(id separator ',')
from (
select user_id as id from t
union
select req_user_id from t
) x
| group_concat(id separator ',') |
|---|
| 1,2,7,9 |