面试题：怎么把这样一个表儿
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 t m where month='1' and m.year = t.year) as m1, (select amount from t m where month='2' and m.year = t.year) as m2, (select amount from t m where month='3' and m.year = t.year) as m2, (select amount from t m where month='4' and m.year = t.year) as m2 from t group by year