SQL分组多列统计(GROUP BY后按条件分列统计) -转

今天在园子里看到个group by 分组多列统计的例子，转走给大家分享一下：

create table tests (year datetime year to year,type char(1),value int);
alter table tests alter colomn year int;

insert into tests values (2015,1,100);
insert into tests values (2015,2,200);
insert into tests values (2016,1,150);
insert into tests values (2016,2,300);
insert into tests values (2016,3,100);

YEAR	TYPE	VALUE
2015	1	100
2015	2	200
2016	1	150
2016	2	300
2016	3	100

转为：

YEAR	TYPE1	TYPE2	TYPE3
2015	100	200	0
2016	150	300	100

这时候我们除了用到GROUP BY之外还需要CASE WHEN，SQL如下：

SELECT year,
SUM(CASE WHEN type=1 THEN value ELSE 0 END) as type1,
SUM(CASE WHEN type=2 THEN value ELSE 0 END) as type2,
SUM(CASE WHEN type=3 THEN value ELSE 0 END) as type3,
FROM table_test GROUP BY year