SQL统计每年每月的信息

                  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
                  提示：这个与工资条非常类似，与学生的科目成绩也很相似。

 create table sales(id int auto_increment primary key,year
                  varchar(10), month varchar(10), amount float(2,1));
                  insert into sales values
                  (null,'1991','1',1.1),
                  (null,'1991','2',1.2),
                  (null,'1991','3',1.3),
                  (null,'1991','4',1.4),
                  (null,'1992','1',2.1),
                  (null,'1992','2',2.2),
                  (null,'1992','3',2.3),
                  (null,'1992','4',2.4);

 

 

sql：        select sales.year ,
                  (select t.amount from sales t where t.month='1' and t.year=
                  sales.year) '1',
                  (select t.amount from sales t where t.month='1' and t.year=
                  sales.year) '2',
                  (select t.amount from sales t where t.month='1' and t.year=
                  sales.year) '3',
                  (select t.amount from sales t where t.month='1' and t.year=
                  sales.year) as '4'
                  from sales group by year;