mysql 取每组前几条记录
1.用子查询：
SELECT * FROM tableName a WHERE 3>
(SELECT COUNT(*) FROM tableName b WHERE b.id=a.id AND b.cnt>a.cnt)
ORDER BY a.id,a.account DESC
2.用exists半连接：
SELECT * FROM tableName a WHERE EXISTS
(SELECT COUNT(*) FROM tableName b WHERE b.id=a.id AND a.account<b.cnt HAVING COUNT(*)<3)
ORDER BY a.id,a.cntDESC
二、同理可以取组内最小的N条记录：例如：取每组的前3条最小的记录
１、子查询
SELECT * FROM tableName a WHERE 3>
(SELECT COUNT(*) FROM tableName b WHERE b.id=a.id AND b.cnt<a.cnt)
ORDER BY a.id,a.cnt DESC
２、用exists：
SELECT * FROM tableName a WHERE EXISTS
(SELECT COUNT(*) FROM tableName b WHERE b.id=a.id AND a.cnt>b.cnt HAVING COUNT(*)<3)
ORDER BY a.id,a.cnt DESC
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