用一条sql语句实现下面结果：
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  

以下答案，可能表名不同。
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我的答案1：
SQL> select year,
max(decode(month,1,amount)) as m1,
max(decode(month,2,amount)) as m2,
max(decode(month,3,amount)) as m3,
max(decode(month,4,amount)) as m4
from mrtest
group by year;
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我的答案2：
SQL> select  a.year year,a.amount m1,b.amount m2,c.amount m3,d.amount m4 from
mrtest a,mrtest b,mrtest c,mrtest d
where a.month=1 and b.month=2 and c.month=3 and d.month=4 and a.year=b.year
and b.year=c.year and c.year=d.year;
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其它答案3：
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
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答案5：
select year,
sum(case when month = '1' then amount else '0' end) as m1,
sum(case when month = '2' then amount else '0' end) as m2,
sum(case when month = '3' then amount else '0' end) as m3,
sum(case when month = '4' then amount else '0' end) as m4
from table_name
group by year