MySQL-分组排序后取前3条

EXPLAIN SELECT a.*
FROM
(
SELECT a.*, IF(@tmpgid = strategy_stockid, @rank := @rank+1, @rank := 0) AS rank, 
@tmpgid := strategy_stockid, @number := @number+1, @rank, @tmpgid
FROM
(SELECT * FROM web_strategy_stockdetail ORDER BY strategy_stockid, high_profit DESC) a, 
(SELECT @rank :=0, @number :=0, @tmpgid := '') b) a
WHERE rank < 3;




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