好文收藏-mysql分组取每组前几条记录(排序)

测试数据 [sql] ​

1. CREATETABLE`mygoods` (
2. `goods_id`int(11) unsigned NOT NULL AUTO_INCREMENT,
3. `cat_id`int(11) NOT NULLDEFAULT'0',
4. `price` tinyint(3) NOT NULLDEFAULT'0',
5. `status` tinyint(3)DEFAULT'1',
6. PRIMARYKEY(`goods_id`),
7. KEY`icatid` (`cat_id`)
8. ) ENGINE=InnoDBDEFAULTCHARSET=utf8;
9. 
   
10. INSERTINTO`mygoods`VALUES(1, 101, 90, 0);
11. INSERTINTO`mygoods`VALUES(2, 101, 99, 1);
12. INSERTINTO`mygoods`VALUES(3, 102, 98, 0);
13. INSERTINTO`mygoods`VALUES(4, 103, 96, 0);
14. INSERTINTO`mygoods`VALUES(5, 102, 95, 0);
15. INSERTINTO`mygoods`VALUES(6, 102, 94, 1);
16. INSERTINTO`mygoods`VALUES(7, 102, 93, 1);
17. INSERTINTO`mygoods`VALUES(8, 103, 99, 1);
18. INSERTINTO`mygoods`VALUES(9, 103, 98, 1);
19. INSERTINTO`mygoods`VALUES(10, 103, 97, 1);
20. INSERTINTO`mygoods`VALUES(11, 104, 96, 1);
21. INSERTINTO`mygoods`VALUES(12, 104, 95, 1);
22. INSERTINTO`mygoods`VALUES(13, 104, 94, 1);
23. INSERTINTO`mygoods`VALUES(15, 101, 92, 1);
24. INSERTINTO`mygoods`VALUES(16, 101, 93, 1);
25. INSERTINTO`mygoods`VALUES(17, 101, 94, 0);
26. INSERTINTO`mygoods`VALUES(18, 102, 99, 1);
27. INSERTINTO`mygoods`VALUES(19, 105, 85, 1);
28. INSERTINTO`mygoods`VALUES(20, 105, 89, 0);
29. 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 |
+----------+--------+-------+--------+

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)