/* BAD:   result: 3 */
SELECT COUNT(*)
FROM t1
WHERE EXISTS(SELECT * FROM t2 WHERE t2.t1id = t1.id);

/* GOOD:   result: 2 */
SELECT COUNT(id)
FROM t1
WHERE EXISTS(SELECT * FROM t2 WHERE t2.t1id = t1.id);

/* GOOD:   result: 2 rows */
SELECT *
FROM t1
WHERE EXISTS(SELECT * FROM t2 WHERE t2.t1id = t1.id);