遍历Postgres中的列

Question

我的桌子像：

Name  Stages               Year  Units
P1    Under Construction   2015   10
P2    Completed            2016   20
P3    Under Construction   2015   30
P4    Completed            2016   40
P5    Under Construction   2016   50

我的输出看起来应该像这样：

Stages               2015  2016
Under Construction    40    50
Completed              0    60 

我尝试了以下查询：

select sum(units) from table_1 where
stages = 'Under Construction' and year = 2016 ;

Answer 1

只需使用条件聚合：

select stages, 
       sum(case when year = 2015 then units else 0 end) from units_2015,
       sum(case when year = 2016 then units else 0 end) from units_2016
from t
group by stages;