有时需求，对海量数据中按条件查询，但只要前若干行数据的话，适合本SQL算法；

SET @rn = 0;
SET @[要限定的字段变量]= '';

SELECT 
	*, @rn:= if(@[要限定的字段变量]=`字段名`, @rn + 1, 1) AS `计数`,
	@[字段名]:= `[字段名]` AS `字段内容项`,
	@rn AS `inner_row_num`
FROM 
	`[表名]`
GROUP BY
	`[字段名]`,
	`inner_row_num`
HAVING `inner_row_num` <= [限定的行数];