EXPLAIN SYNTAX
SELECT 1
UNION ALL
(
    SELECT 1
    UNION ALL
    (
        SELECT 1
        UNION ALL
        SELECT 1
    )
    UNION ALL
    SELECT 1
);

SELECT ' ';

EXPLAIN SYNTAX
SELECT 1
UNION ALL
(
    SELECT 1
    UNION DISTINCT
    (
        SELECT 1
        UNION ALL
        SELECT 1
    )
    UNION ALL
    SELECT 1
);

SELECT ' ';

EXPLAIN SYNTAX
SELECT x
FROM
(
    SELECT 1 AS x
    UNION ALL
    (
        SELECT 1
        UNION DISTINCT
        (
            SELECT 1
            UNION ALL
            SELECT 1
        )
        UNION ALL
        SELECT 1
    )
);

SELECT ' ';

EXPLAIN SYNTAX
SELECT x
FROM 
(
    SELECT 1 AS x
    UNION ALL
    (
        SELECT 1
        UNION ALL
        SELECT 1
    )
);

SELECT ' ';

EXPLAIN SYNTAX
SELECT 1
UNION ALL
SELECT 1
UNION DISTINCT
SELECT 1;

SELECT ' ';

EXPLAIN SYNTAX
(((((((((((((((SELECT 1)))))))))))))));

SELECT ' ';

EXPLAIN SYNTAX
(((((((((((((((SELECT 1 UNION DISTINCT SELECT 1))) UNION DISTINCT SELECT 1)))) UNION ALL SELECT 1))))))));