DROP TEMPORARY TABLE IF EXISTS tData;
CREATE TEMPORARY TABLE tData
	ENGINE = MEMORY
	SELECT 1 `group`, 1 `max`, 10 `value`
		UNION SELECT 1, 2, 20
		UNION SELECT 1, 3, 30
		UNION SELECT 1, 2, 21
		UNION SELECT 1, NULL, -1
		UNION SELECT 2, '4', 40
		UNION SELECT 2, '6', 60
		UNION SELECT 2, '5', 50
		UNION SELECT 2, '5', 51
		UNION SELECT 2, NULL, -1
		UNION SELECT 3, 7, 70
		UNION SELECT 3, 8, 80
		UNION SELECT 3, 9, NULL
		UNION SELECT 3, 8, 81
		UNION SELECT 3, NULL, -1
		UNION SELECT 4, NULL, 101
		UNION SELECT 4, NULL, 102;

DROP TEMPORARY TABLE IF EXISTS tExpect;
CREATE TEMPORARY TABLE tExpect
	ENGINE = MEMORY
	SELECT 1 `group`, 30 expect
		UNION SELECT 2, 60
		UNION SELECT 3, NULL
		UNION SELECT 4, 102;

SELECT e.`group`, r.result, e.expect, r.result <=> e.expect isRight
	FROM tExpect e
		LEFT JOIN (
			SELECT `group`, multimax(`max`, `value`) result
				FROM tData GROUP BY `group`
		) r ON r.`group` = e.`group`;

DROP TEMPORARY TABLE tData, tExpect;