Wrong result with DISTINCT and GROUP BY when STD() is wrapped in CASE

A SELECT DISTINCT query with GROUP BY returns an incorrect result when STD(v) is replaced with the equivalent expression CASE WHEN 1 THEN STD(v) ELSE STD(v) END.

The original query returns two groups. The modified query returns only one row, combining the grouping value from the first group with the average from the second group.

Reproducer

CREATE TABLE t (id INT, v INT);

INSERT INTO t VALUES (1,0),(1,1),(1,2),(2,10),(2,11),(2,12);

SELECT DISTINCT

  CASE WHEN (SELECT 1) THEN id ELSE id END AS grp,

  AVG(v) AS avg_v,

  STD(v) AS std_v

FROM t GROUP BY id;

SELECT DISTINCT

  CASE WHEN (SELECT 1) THEN id ELSE id END AS grp,

  AVG(v) AS avg_v,

  CASE WHEN 1 THEN STD(v) ELSE STD(v) END AS std_v

FROM t GROUP BY id;

Actual result

The first query returns:

+------+---------+--------+

| grp  | avg_v   | std_v  |

+------+---------+--------+

|    1 |  1.0000 | 0.8165 |

|    2 | 11.0000 | 0.8165 |

+------+---------+--------+

The second query returns:

+------+---------+--------+

| grp  | avg_v   | std_v  |

+------+---------+--------+

|    1 | 11.0000 | 0.8165 |

+------+---------+--------+

The returned row has grp = 1, but avg_v = 11.0000 is the average for id = 2.

Expected result

Both queries should return the same two rows as the first query, ignoring row order.

CASE WHEN 1 THEN STD(v) ELSE STD(v) END is equivalent to STD(v). CASE WHEN (SELECT 1) THEN id ELSE id END is equivalent to id. Since the grouping values differ, DISTINCT should retain both rows.

Additional observations

• Removing DISTINCT from the second query restores the expected two rows.
• Replacing the first selected CASE expression with id also restores the expected result.
• Only MariaDB 11.8.9 has been verified. The source-level cause has not been determined.