CREATE TABLE `mygoods` (  `goods_id` int(11) unsigned NOT NULL AUTO_INCREMENT,  `cat_id` int(11) NOT NULL DEFAULT '0',  `price` tinyint(3) NOT NULL DEFAULT '0',  `status` tinyint(3) DEFAULT '1',  PRIMARY KEY (`goods_id`),  KEY `icatid` (`cat_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8;  INSERT INTO `mygoods` VALUES (1, 101, 90, 0);
INSERT INTO `mygoods` VALUES (2, 101, 99, 1);
INSERT INTO `mygoods` VALUES (3, 102, 98, 0);
INSERT INTO `mygoods` VALUES (4, 103, 96, 0);
INSERT INTO `mygoods` VALUES (5, 102, 95, 0);
INSERT INTO `mygoods` VALUES (6, 102, 94, 1);
INSERT INTO `mygoods` VALUES (7, 102, 93, 1);
INSERT INTO `mygoods` VALUES (8, 103, 99, 1);
INSERT INTO `mygoods` VALUES (9, 103, 98, 1);
INSERT INTO `mygoods` VALUES (10, 103, 97, 1);
INSERT INTO `mygoods` VALUES (11, 104, 96, 1);
INSERT INTO `mygoods` VALUES (12, 104, 95, 1);
INSERT INTO `mygoods` VALUES (13, 104, 94, 1);
INSERT INTO `mygoods` VALUES (15, 101, 92, 1);
INSERT INTO `mygoods` VALUES (16, 101, 93, 1);
INSERT INTO `mygoods` VALUES (17, 101, 94, 0);
INSERT INTO `mygoods` VALUES (18, 102, 99, 1);
INSERT INTO `mygoods` VALUES (19, 105, 85, 1);
INSERT INTO `mygoods` VALUES (20, 105, 89, 0);
INSERT INTO `mygoods` VALUES (21, 105, 99, 1);  

mysql> select a.*   -> from mygoods a   -> where (select count(*) -> from mygoods -> where cat_id = a.cat_id and price > a.price  ) <2 -> order by a.cat_id,a.price desc;

+----------+--------+-------+--------+
| goods_id | cat_id | price | status |
+----------+--------+-------+--------+
|        2 |    101 |    99 |      1 |
|       17 |    101 |    94 |      0 |
|       18 |    102 |    99 |      1 |
|        3 |    102 |    98 |      0 |
|        8 |    103 |    99 |      1 |
|        9 |    103 |    98 |      1 |
|       11 |    104 |    96 |      1 |
|       12 |    104 |    95 |      1 |
|       21 |    105 |    99 |      1 |
|       20 |    105 |    89 |      0 |
+----------+--------+-------+--------+
10 rows in set (0.00 sec)

2.每个分类找出价格最高的有效的两个商品（正确）

mysql> select a.* -> from mygoods a -> where (select count(*) from mygoods -> where cat_id = a.cat_id and price > a.price and status=1  ) <2 -> and status=1 -> order by a.cat_id,a.price desc ;

+----------+--------+-------+--------+
| goods_id | cat_id | price | status |
+----------+--------+-------+--------+
|        2 |    101 |    99 |      1 |
|       16 |    101 |    93 |      1 |
|       18 |    102 |    99 |      1 |
|        6 |    102 |    94 |      1 |
|        8 |    103 |    99 |      1 |
|        9 |    103 |    98 |      1 |
|       11 |    104 |    96 |      1 |
|       12 |    104 |    95 |      1 |
|       21 |    105 |    99 |      1 |
|       19 |    105 |    85 |      1 |
+----------+--------+-------+--------+
10 rows in set (0.00 sec)

mysql> select a.* -> from mygoods a -> left join mygoods b -> on a.cat_id = b.cat_id and a.price < b.price and b.status=1-> where a.status=1-> group by a.goods_id,a.cat_id,a.price-> having count(b.goods_id) < 2-> order by a.cat_id,a.price desc;

+----------+--------+-------+--------+
| goods_id | cat_id | price | status |
+----------+--------+-------+--------+
|        2 |    101 |    99 |      1 |
|       16 |    101 |    93 |      1 |
|       18 |    102 |    99 |      1 |
|        6 |    102 |    94 |      1 |
|        8 |    103 |    99 |      1 |
|        9 |    103 |    98 |      1 |
|       11 |    104 |    96 |      1 |
|       12 |    104 |    95 |      1 |
|       21 |    105 |    99 |      1 |
|       19 |    105 |    85 |      1 |
+----------+--------+-------+--------+
10 rows in set (0.00 sec)

mysql> select a.* -> from mygoods a -> where (select count(*) from mygoods -> where cat_id = a.cat_id and price > a.price  ) <2 and status=1 -> order by a.cat_id,a.price desc;

+----------+--------+-------+--------+
| goods_id | cat_id | price | status |
+----------+--------+-------+--------+
|        2 |    101 |    99 |      1 |
|       18 |    102 |    99 |      1 |
|        8 |    103 |    99 |      1 |
|        9 |    103 |    98 |      1 |
|       11 |    104 |    96 |      1 |
|       12 |    104 |    95 |      1 |
|       21 |    105 |    99 |      1 |
+----------+--------+-------+--------+
7 rows in set (0.00 sec)

mysql> select cat_id,GROUP_CONCAT(goods_id) from mygoods group by cat_id;

+--------+------------------------+
| cat_id | GROUP_CONCAT(goods_id) |
+--------+------------------------+
|    101 | 1,2,15,16,17           |
|    102 | 3,5,6,7,18             |
|    103 | 4,8,9,10               |
|    104 | 11,12,13               |
|    105 | 19,20,21               |
+--------+------------------------+
5 rows in set (0.00 sec)

1.用子查询： SELECT*
FROMright2 a
WHERE2 > (SELECTCOUNT(*)FROMright2 bWHEREb.id = a.idAND b.account > a.account)
ORDER BYa.id,a.account DESC 2.用exists半连接： SELECT*FROMright2 aWHEREEXISTS (SELECTCOUNT(*)FROMright2 bWHEREb.id = a.idAND a.account < b.accountHAVINGCOUNT(*) < 2)ORDER BYa.id,a.account DESC 同理可以取组内最小的N条记录： SELECT*FROMright2 aWHERE2 > (SELECTCOUNT(*)FROMright2 bWHEREb.id = a.idAND b.account < a.account)ORDER BYa.id,a.account DESC 用exists： SELECT*FROMright2 aWHEREEXISTS (SELECTCOUNT(*)FROMright2 bWHEREb.id = a.idAND a.account > b.accountHAVINGCOUNT(*) < 2)ORDER BYa.id,a.account DESC SQLServer支持top - N： SELECTa.*FROMtb aWHEREval = (SELECTtop 3 valFROMtbWHERENAME = a. NAME)ORDER BYa. NAME

SELECTid,val
FROMt b
INNER JOIN (SELECT*FROMt aWHEREORDER BYval DESC
) a ON a.id = b.id
GROUP BYa.id
ORDER BYid;

SELECT*
FROM(SELECT*FROMt2ORDER BYcol2 DESC) AS a
GROUP BYgid
ORDER BYgid;

SELECT*
FROMt2 a
WHERENOT EXISTS (SELECT1FROMt2 bWHEREb.gid = a.gidAND b.col2 > a.col2)
ORDER BYa.gid;

SELECT*
FROMt2 a
WHERE3 > (SELECTcount(*)FROMt2WHEREgid = a.gidAND col2 > a.col2)
ORDER BYa.gid,a.col2 DESC;

SELECT*
FROMt2 a
WHEREEXISTS (SELECTcount(*)FROMt2 bWHEREb.gid = a.gidAND a.col2 < b.col2HAVING(count(*)) < 3)
ORDER BYa.gid,a.col2 DESC

SELECT * FROM(
SELECT z.type , z.code ,ROW_NUMBER()
OVER(PARTITION BY z.type ORDER BY z.code desc) AS code_id
FROM group_info z
)
WHERE code_id <4;   #取每组最大的前四条记录