[Oracle]大话Oracle中的NULL
->null与in:null值不会包含在in的结果集中。
A105024@O02DMS1>select * from test;
ID NAME
---------- --------------------
1 A
B
3 C
A105024@O02DMS1>select * from test where id in (1);
ID NAME
---------- --------------------
1 A
A105024@O02DMS1>select * from test where id not in(1);
ID NAME
---------- --------------------
3 C
从上面的例子我们可以看出,不管是in还是not in,null值都不在结果集中,除非显示的声明:
A105024@O02DMS1>select * from test where id is null;
ID NAME
---------- --------------------
B
-->null与“加减乘除等于”的关系:null不能用来进行等于比较;与任何东西的加减乘除的结果都是null。
A105024@O02DMS1>select * from test where id = null;
no rows selected
A105024@O02DMS1>select id+1 from test where id is null;
ID+1
----------
-->null与集合运算的关系:集合运算中将所有的null作为相等的值来对待。
A105024@O02DMS1>select null from dual union select null from dual;
N
-
A105024@O02DMS1>select null from dual minus select null from dual;
no rows selected
-->null与Group by 的关系:group by把null作为一个值来对待。
A105024@O02DMS1>select id,count(*) from test group by id;
ID COUNT(*)
---------- ----------
1 1
1
3 1
-->null与Order by的关系:order by 默认排序规则是将空值放在最后(不管是升序还是降序),除非在order by后面加上nulls first。
A105024@O02DMS1>select * from test order by id;
ID NAME
---------- --------------------
1 A
3 C
B
A105024@O02DMS1>select * from test order by id desc;
ID NAME
---------- --------------------
B
3 C
1 A
A105024@O02DMS1>select * from test order by id nulls first;
ID NAME
---------- --------------------
B
1 A
3 C
-->null与聚合函数的关系:聚合函数如sum,avg,min,max将忽略null值,一个例外是count,count(*)包含null值,而count(列名)不包含null值。
A105024@O02DMS1>select count(*) from test;
COUNT(*)
----------
3
A105024@O02DMS1>select count(id) from test;
COUNT(ID)
----------
2