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
十步理解SQL：http://blog.jobbole.com/55086/
