mysql分组取每组前几条记录(排序)
mysql分组取每组前几条记录(排名) 附group by与order by的研究
测试数据 [sql]
- CREATETABLE`mygoods` (
- `goods_id`int(11) unsigned NOT NULL AUTO_INCREMENT,
- `cat_id`int(11) NOT NULLDEFAULT'0',
- `price` tinyint(3) NOT NULLDEFAULT'0',
- `status` tinyint(3)DEFAULT'1',
- PRIMARYKEY(`goods_id`),
- KEY`icatid` (`cat_id`)
- ) ENGINE=InnoDBDEFAULTCHARSET=utf8;
- INSERTINTO`mygoods`VALUES(1, 101, 90, 0);
- INSERTINTO`mygoods`VALUES(2, 101, 99, 1);
- INSERTINTO`mygoods`VALUES(3, 102, 98, 0);
- INSERTINTO`mygoods`VALUES(4, 103, 96, 0);
- INSERTINTO`mygoods`VALUES(5, 102, 95, 0);
- INSERTINTO`mygoods`VALUES(6, 102, 94, 1);
- INSERTINTO`mygoods`VALUES(7, 102, 93, 1);
- INSERTINTO`mygoods`VALUES(8, 103, 99, 1);
- INSERTINTO`mygoods`VALUES(9, 103, 98, 1);
- INSERTINTO`mygoods`VALUES(10, 103, 97, 1);
- INSERTINTO`mygoods`VALUES(11, 104, 96, 1);
- INSERTINTO`mygoods`VALUES(12, 104, 95, 1);
- INSERTINTO`mygoods`VALUES(13, 104, 94, 1);
- INSERTINTO`mygoods`VALUES(15, 101, 92, 1);
- INSERTINTO`mygoods`VALUES(16, 101, 93, 1);
- INSERTINTO`mygoods`VALUES(17, 101, 94, 0);
- INSERTINTO`mygoods`VALUES(18, 102, 99, 1);
- INSERTINTO`mygoods`VALUES(19, 105, 85, 1);
- INSERTINTO`mygoods`VALUES(20, 105, 89, 0);
- INSERTINTO`mygoods`VALUES(21, 105, 99, 1);
说明：
表mygoods为商品表，cat_id为分类id，goods_id为商品id，status为商品当前的状态位（1:有效，0:无效）。
需求：每个分类下，找出两个价格最高的有效的商品。
1.每个分类找出价格最高的两个商品
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) 3.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 -> 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) 4.每个分类找出价格最高的有效的两个商品（错误） 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) 由上可知，如果需要增加条件的话，需要在两处增加条件。 可以将每个分组下的goods_id合并。 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)
