看到很多人对于keep不理解，这里解释一下！
Returns the row ranked first using DENSE_RANK
2种取值：
DENSE_RANK FIRST
DENSE_RANK LAST
在keep (DENSE_RANK first ORDER BY sl) 结果集中再取max、min的例子。
SQL> select * from test;
ID MC SL
-------------------- -------------------- -------------------
1 111 1
1 222 1
1 333 2
1 555 3
1 666 3
2 111 1
2 222 1
2 333 2
2 555 2
9 rows selected
SQL>
SQL> select id,mc,sl,
2 min(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id),
3 max(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id)
4 from test
5 ;
ID MC SL MIN(MC)KEEP(DENSE_RANKFIRSTORD MAX(MC)KEEP(DENSE_RANKLASTORDE
-------------------- -------------------- ------------------- ------------------------------ ------------------------------
1 111 1 111 666
1 222 1 111 666
1 333 2 111 666
1 555 3 111 666
1 666 3 111 666
2 111 1 111 555
2 222 1 111 555
2 333 2 111 555
2 555 2 111 555
9 rows selected
SQL>
不要混淆keep内（first、last）外（min、max或者其他）：
min是可以对应last的
max是可以对应first的
SQL> select id,mc,sl,
2 min(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id),
3 max(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id),
4 min(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id),
5 max(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id)
6 from test
7 ;
ID MC SL MIN(MC)KEEP(DENSE_RANKFIRSTORD MAX(MC)KEEP(DENSE_RANKFIRSTORD MIN(MC)KEEP(DENSE_RANKLASTORDEMAX(MC)KEEP(DENSE_RANKLASTORDE
-------------------- -------------------- ------------------- ------------------------------ ------------------------------ ------------------------------ ------------------------------
1 111 1 111 222 555 666
1 222 1 111 222 555 666
1 333 2 111 222 555 666
1 555 3 111 222 555 666
1 666 3 111 222 555 666
2 111 1 111 222 333 555
2 222 1 111 222 333 555
2 333 2 111 222 333 555
2 555 2 111 222 333 555
9 rows selected
SQL> select id,mc,sl,
2 min(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id),
3 max(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id),
4 min(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id),
5 max(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id)
6 from test
7 ;
ID MC SL MIN(MC)KEEP(DENSE_RANKFIRSTORD MAX(MC)KEEP(DENSE_RANKFIRSTORD MIN(MC)KEEP(DENSE_RANKLASTORDEMAX(MC)KEEP(DENSE_RANKLASTORDE
-------------------- -------------------- ------------------- ------------------------------ ------------------------------ ------------------------------ ------------------------------
1 111 1 111 222 555 666
1 222 1 111 222 555 666
1 333 2 111 222 555 666
1 555 3 111 222 555 666
1 666 3 111 222 555 666
2 111 1 111 222 333 555
2 222 1 111 222 333 555
2 333 2 111 222 333 555
2 555 2 111 222 333 555
min(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id)：id等于1的数量最小的（DENSE_RANK first ）为
1 111 1
1 222 1
在这个结果中取min(mc) 就是111
max(mc) keep (DENSE_RANK first ORDER BY sl) over(partition by id)
取max(mc) 就是222；
min(mc) keep (DENSE_RANK last ORDER BY sl) over(partition by id)：id等于1的数量最大的（DENSE_RANK first ）为
1 555 3
1 666 3
在这个结果中取min(mc) 就是222，取max(mc)就是666