|id_a | id_b |
|  1  |   1  |
|  1  |   2  |
|  1  |   3  |
|  2  |   1  |
|  2  |   4  |
|  3  |   1  |
|  3  |   2  |

|id_a | id_b |
|  1  |   1, 2, 3  |
|  2  |   1, 4     |
|  3  |   1,2      |

SELECT DISTINCT t1.id_a
FROM test t1 INNER JOIN (SELECT DISTINCT id_a FROM test WHERE id_b = 1) t2
     ON t1.id_a = t2.id_a
WHERE t1.id_b = 2

SELECT id_a FROM table WHERE id_b in (1, 2) GROUP BY id_a;