SELECT * FROM test_table t, test_table2 t2
WHERE
t.id = 50 AND t2.t1_id = 50


SELECT * FROM test_table t, test_table2 t2
WHERE
t.id = 50 AND t2.t1_id = t1.id


(assuming that table t has one column: id (pkey) and table t2 has two columns id (pkey) and t1_id (fkey))