mysql 最終版解決分組topN方案

SELECT
		name,
		age,
		itemTime
	FROM
		(
			SELECT
				csg.*, CASE
			WHEN @mid = csg.itemTime THEN
				@rownum :=@rownum + 1
			ELSE
				@rownum := 1
			END rownum,
			@mid := csg.itemTime MID
		FROM

		(SELECT @rownum := 0) r,
		(SELECT @mid := 0) m,
			(
				SELECT
					id,
					DATE_FORMAT(
						date_add(dataTime, INTERVAL 1 HOUR),
						'%Y-%m-%d %H'
					) AS itemTime,
					name,
					age
				FROM
					table w
				WHERE
					cty= 123
				ORDER BY
					itemTime,
					dataTime DESC
			) csg
		) t
	WHERE
		t.rownum = 1
	ORDER BY
		t.itemTime DESC

公司最近遇到分組後取前十的 由於mysql 沒有topN 算法 要自己寫