1、有一个数据库表aaa，
year month amount
1991 1 1.1
1991 2 1.2
1991 3 1.3
1991 4 1.4
1992 1 2.1
1992 2 2.2
1992 3 2.3
1992 4 2.4
怎样查成这样一个结果
year m1 m2 m3 m4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4
解：
select year,
(select amount from aaa m where month=1 and
m.year=aaa.year) as m1,
(select amount from aaa m where month=2 and
m.year=aaa.year) as m2,
(select amount from aaa m where month=3 and m.year=aaa.year)
as m3,
(select amount from aaa m where month=4 and
m.year=aaa.year) as m4
from aaa group by year
2、
| pid | type | resource |
| 1337 | Memory | 1020 |
| 1337 | CPU | 70 |
| 1433 | Memory | 2020 |
| 1433 | CPU | 60 |
要变成
|
pid |
Memory | CPU |
| 1337 | 1020 | 70 |
| 1433 | 2020 | 60 |
解：
select pid
(select resource from table m where type=Memory and table.pid=m.pid) as Memory,
(select resource from table m where type=CPU and table.pid=m.pid) as CPU
from table
group by pid
后续：
查看from table m的语法使用