进行分组和转换查询 Oracle

Question

我有一张测试表：
category   type  quantities
a      1    100
a      2    150
b      2    45
b      3    68
b      1    72
c      2    90
c      3    39

假设只出现了 3 种类型。我的目标是选择各种类别和每种类型的数量。结果应该是：

类别   type1   type2   type3
a      100    150    0
b      72     45    68
c      0     90    39

我正在尝试联合，但我认为它是多余的而不是简短的：

select category, sum(type1)type1, sum(type2)type2 ,sum(type3) type3 from 
(
select category, sum(quantities) type1, 0 type2, 0 type3  from test where type=1 group by category
union all
select category, 0 type1, sum(quantities) type2, 0 type3  from test where type=2 group by category
union all
select category, 0 type1, 0 type2, sum(quantities) type3 from test where type=3 group by category
) group by category;

我可以做些什么来缩短我的查询？感谢您的任何建议。

Answer 1

那是简单的条件聚合：

select 
  category,
  nvl(sum(case when type = 1 then quantities end), 0) as type1,
  nvl(sum(case when type = 2 then quantities end), 0) as type2,
  nvl(sum(case when type = 3 then quantities end), 0) as type3
from mytable
group by category
order by category;

Answer 2

也称为数据透视表——参见http://www.oracle.com/technetwork/articles/sql/11g-pivot-097235.html