oracle 11g pivot unpivot 实现行列抓换

表YEA如下图所示：
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
 请将表YEA转置，按如下方式显示
YEAR M1 M2 M3 M4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4


select * from yea pivot(sum(amount) for month in ('1' m1,'2' m2,'3' m3,'4'm4));