高效的查询到组，并把所有其他领域中的行SQL

Question

试想一个查询的结果是类似如下：

+----+---------+--------+ 
| id | count | type | 
+----+---------+--------+ 
| 1 | 20  | a  | 
| 1 | 30  | b  | 
| 1 | 10  | c  | 
| 2 | 05  | a  | 
| 2 | 20  | b  | 
| 2 | 40  | c  | 
+----+---------+--------+ 

和预期的结果：

+----+---------+--------+------+ 
| id | a  | b  | c | 
+----+---------+--------+------+ 
| 1 | 20  | 30  | 10 | 
| 2 | 05  | 20  | 40 | 
+----+---------+--------+------+ 

我知道一些使用Cursor，Variables，Join等很复杂的解决方案。我想找到最高效的解决方案，否则我会从应用程序层处理它。

Answer 1

一种方法是使用有条件聚集：

select id, 
     sum(case when type = 'a' then count else 0 end) as a, 
     sum(case when type = 'b' then count else 0 end) as b, 
     sum(case when type = 'c' then count else 0 end) as c 
from t 
group by id;