一道SQL面试题

怎么把这样一个表

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

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

答案：

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