Question

I want to use an array of (id_1)ids from a selection on table_1 to get all rows from table_2 where id_2 = id_1

  SELECT field_b
  FROM table_2
  WHERE id_2 IN (
         SELECT id_1 FROM table_1 WHERE field_a = 1234
  );

Below are example of my tables for reference:

  mysql> select * from table_1;
    +-------+---------+
    |  id_1 | field_a |
    +-------+---------+
    |     1 |    1234 |
    +-------+---------+
    |     2 |    1234 |
    +-------+---------+


mysql> select * from table_2;
+---------+---------+
|    id_2 | field_b |
+---------+---------+
|     1   |  5678   |
+-------+-----------+
|     2   |  0013   |
+-------+-----------+

Answer 1

Would an INNER JOIN work just as well well for you?

SELECT field_b
FROM table_2
INNER JOIN table_1
    ON table_2.id_2 = table_1.id_1
WHERE table_2.field_a=1234