我们都知道在Oracle SQL语句中order by 是用来排序查询出来的结果集的，而在Oracle中NULL值是一个很特殊的值，如果order by指定的列有NULL值，那排序结果又是怎样的呢。
下面做一组实验观察一下order by时Oracle是怎么处理NULL的
版本11.2.0.4
1、创建测试表并插入测试数据
linuxidc@ORCL>create table t (id number,name varchar2(10));
Table created.
linuxidc@ORCL>insert into t values(1,'zx');
1 row created.
linuxidc@ORCL>insert into t values(2,'wl');
1 row created.
linuxidc@ORCL>insert into t values(3,'zxt');
1 row created.
linuxidc@ORCL>insert into t values(4,NULL);
1 row created.
linuxidc@ORCL>insert into t values(5,'yhz');
1 row created.
linuxidc@ORCL>insert into t values(6,NULL);
1 row created.
linuxidc@ORCL>commit;
Commit complete.
linuxidc@ORCL>select * from t;
ID NAME
---------- ------------------------------
1 zx
2 wl
3 zxt
4
5 yhz
6
6 rows selected.
2、测试order by
linuxidc@ORCL>select * from t order by name asc;
ID NAME
---------- ------------------------------
2 wl
5 yhz
1 zx
3 zxt
6
4
6 rows selected.
linuxidc@ORCL>select * from t order by name desc;
ID NAME
---------- ------------------------------
4
6
3 zxt
1 zx
5 yhz
2 wl
6 rows selected.
看到不同的排序方式，NULL值所排序的位置不同。升序(asc)NULL排在最后，降序(desc)NULL排在最前。
我们再来看看官方文档是怎么描述的
ASC | DESC Specify the ordering sequence (ascending or descending). ASC is the default.
NULLS FIRST | NULLS LAST Specify whether returned rows containing nulls should appear first or last in the ordering sequence.
NULLS LAST is the default for ascending order, and NULLS FIRST is the default for descending order.
可以看到我们的实验结果与官方文档描述是一致的。而且还可以使用NULLS FIRST|NULLS LAST来决定NULL的值是排在最前还是排在最后。
3、再次做实验验证
linuxidc@ORCL>select * from t order by name asc nulls first;
ID NAME
---------- ------------------------------
6
4
2 wl
5 yhz
1 zx
3 zxt
6 rows selected.
linuxidc@ORCL>select * from t order by name asc nulls last;
ID NAME
---------- ------------------------------
2 wl
5 yhz
1 zx
3 zxt
6
4
6 rows selected.
linuxidc@ORCL>select * from t order by name desc nulls first;
ID NAME
---------- ------------------------------
4
6
3 zxt
1 zx
5 yhz
2 wl
6 rows selected.
linuxidc@ORCL>select * from t order by name desc nulls last;
ID NAME
---------- ------------------------------
3 zxt
1 zx
5 yhz
2 wl
6
4
6 rows selected.