oracle keep

语法：

min | max(column1) keep (dense_rank first | last order by column2) over (partion by column3);
--返回按照column3分组后，按照column2排序的结果集中第一个或最后一个最小值或最大值column1。

创建测试数据：

create table test(
id     number,
a      number,
b      number
);

insert into test values (1,111,1 );
insert into test values (1,222,1 );
insert into test values (1,333,1 );
insert into test values (1,555,3 );
insert into test values (1,666,3 );
insert into test values (2,111,1 );
insert into test values (2,222,1 );
insert into test values (2,333,2 );
insert into test values (2,555,2 );
commit;