How to get record in unique array Mysql

Question

I had a table:

----------------------
| user_id | marker_id|
----------------------
| 1 | 1 |
--------
| 1 |2 |
--------
| 2 |1 |
--------
| 2 |2 |
--------
| 2 |3 |
--------
| 3 |1 |
--------

I want get unique user_id has marker_id =(1,2). Don't get user_id has marker_id = (1,2,3). How to get user_id = 1 and not include user_id =2.

Answer 1

select distinct user_id 
from table_name 
where marker_id in (1,2)

Answer 2

One method uses group by :

select user_id
from t
group by user_id
having sum(marker_id = 1) > 0 and
       sum(marker_id = 2) > 0 and
       sum(marker_id not in (1, 2)) = 0;

Each condition in the having clause tests one of the values. The first tests for marker_id = 1 and the > 0 says there is at least one. The second similarly tests for marker_id = 2 . The third says that there are no other marker ids.

I call this type of query a "set-within-sets" subquery (you are looking for marker ids for each user). I find that group by / having is a flexible way to solve these queries.