使用SQL获取不同类型的类型的特定数据

这里有一个常见的SQL问题: 如何找到当前的每个程序的最新的日志? 如何从不同类别中找到最常使用的数据?等等. 通常情况这类问题可以被简化为"从不同的类别中选取特定的数据".
这里假设数据库的表结构及其数据如下所示:

+--------+------------+-------+
| type   | variety    | price |
+--------+------------+-------+
| apple  | gala       |  2.79 | 
| apple  | fuji       |  0.24 | 
| apple  | limbertwig |  2.87 | 
| orange | valencia   |  3.59 | 
| orange | navel      |  9.36 | 
| pear   | bradford   |  6.05 | 
| pear   | bartlett   |  2.14 | 
| cherry | bing       |  2.55 | 
| cherry | chelan     |  6.33 | 
+--------+------------+-------+

查询每个类别的最小的值

这里我们先把我们所期望的结果放在这里

+--------+----------+-------+
| type   | variety  | price |
+--------+----------+-------+
| apple  | fuji     |  0.24 | 
| orange | valencia |  3.59 | 
| pear   | bartlett |  2.14 | 
| cherry | bing     |  2.55 | 

当然了,要得到这问题的答案有很多种.这里用两种来解释一下.
一种常见的方式是self-join,

• 首先根据不同的类别进行分组,

select type, min(price) as minprice
from fruits
group by type;
+--------+----------+
| type   | minprice |
+--------+----------+
| apple  |     0.24 | 
| cherry |     2.55 | 
| orange |     3.59 | 
| pear   |     2.14 | 
+--------+----------+

• 第二部是将查询得到的数据重新连接到相同的table中;因为第一部分的查询已经分组了,因此需要将其放在子查询中并将其连接到table中

select f.type, f.variety, f.price
from (
   select type, min(price) as minprice
   from fruits group by type
) as x inner join fruits as f on f.type = x.type and f.price = x.minprice;
+--------+----------+-------+
| type   | variety  | price |
+--------+----------+-------+
| apple  | fuji     |  0.24 | 
| cherry | bing     |  2.55 | 
| orange | valencia |  3.59 | 
| pear   | bartlett |  2.14 | 

这里还有一种方式是使用相关子查询.从效率上将可能有第一点,但是她更加清晰.

select type, variety, price
from fruits
where price = (select min(price) from fruits as f where f.type = fruits.type);
+--------+----------+-------+
| type   | variety  | price |
+--------+----------+-------+
| apple  | fuji     |  0.24 | 
| orange | valencia |  3.59 | 
| pear   | bartlett |  2.14 | 
| cherry | bing     |  2.55 | 

通过了子查询\相关子查询我们可以比较轻松的从大量数据中获取每一种类的数据.接下来我们更进一步

获取每种数据的TOP N数据

通常情况下各种数据库都提供了诸如MAX() MIN()这样的聚合函数来返回单独的一行数据.但是我们这里需要返回多条数据.先来看看第一种实现:

select type, variety, price
from fruits
where price = (select min(price) from fruits as f where f.type = fruits.type)
   or price = (select min(price) from fruits as f where f.type = fruits.type
      and price > (select min(price) from fruits as f2 where f2.type = fruits.type));
+--------+----------+-------+
| type   | variety  | price |
+--------+----------+-------+
| apple  | gala     |  2.79 | 
| apple  | fuji     |  0.24 | 
| orange | valencia |  3.59 | 
| orange | navel    |  9.36 | 
| pear   | bradford |  6.05 | 
| pear   | bartlett |  2.14 | 
| cherry | bing     |  2.55 | 
| cherry | chelan   |  6.33 | 
+--------+----------+-------+

可以看到其实思路很简单,这里用SQL的伪代码来表示一下其实现方式

select type, variety, price
from fruits
where price = 价钱最低的 OR
  price = 价钱最低的 AND price > 价钱最低的

这种方式可以实现,但是可拓展性比较差.如果现在需要查询TOP 3,TOP 4 怎么办?这里还有一种方式实现查询TOP 2
TOP 2 其实质就是 : 价格小于等于第二个便宜的产品.

select type, variety, price
from fruits
where (
   select count(*) from fruits as f
   where f.type = fruits.type and f.price <= fruits.price
) <= 2;