$t1 = SELECT * FROM AS_TABLE([
    <|k: 1, v: 1|>,
    <|k: 2, v: 2|>,
    <|k: 3, v: 3|>,
    <|k: 4, v: 4|>,
    <|k: 5, v: 5|>,
]);

$t2 = SELECT * FROM AS_TABLE([
    <|k:  6, v:  6|>,
    <|k:  7, v:  7|>,
    <|k:  8, v:  8|>,
    <|k:  9, v:  9|>,
    <|k: 10, v: 10|>,
]);

$t3 = SELECT * FROM AS_TABLE([
    <|k: 11, v: 11|>,
    <|k: 12, v: 12|>,
    <|k: 13, v: 13|>,
    <|k: 14, v: 14|>,
    <|k: 15, v: 15|>,
]);

SELECT
    1  IN (  SELECT k FROM $t1  ),
    1  IN (( SELECT k FROM $t1 )),
    1  IN (((SELECT k FROM $t1))),

    1  IN ( SELECT k FROM $t1  UNION  SELECT k FROM $t2 ),
    6  IN ( SELECT k FROM $t1  UNION  SELECT k FROM $t2 ),
    1  IN ( SELECT k FROM $t1  UNION (SELECT k FROM $t2)),
    6  IN ( SELECT k FROM $t1  UNION (SELECT k FROM $t2)),
    1  IN ((SELECT k FROM $t1) UNION  SELECT k FROM $t2 ),
    6  IN ((SELECT k FROM $t1) UNION  SELECT k FROM $t2 ),
    1  IN ((SELECT k FROM $t1) UNION (SELECT k FROM $t2)),
    6  IN ((SELECT k FROM $t1) UNION (SELECT k FROM $t2)),

    1  IN (SELECT k FROM $t1 UNION SELECT k FROM $t2 UNION SELECT k FROM $t3),
    6  IN (SELECT k FROM $t1 UNION SELECT k FROM $t2 UNION SELECT k FROM $t3),
    11 IN (SELECT k FROM $t1 UNION SELECT k FROM $t2 UNION SELECT k FROM $t3),
;