关于null值的小知识
一、null 值参与算数运算的结果为null，不过对于字符串的连接运算就不同了。
SQL> select 5+2-3 as result1,
2 5+2-3+null as result2,
3 3*5*null as result3
4 from dual;
RESULT1 RESULT2 RESULT3
---------- ---------- ----------
4
二、在聚集函数中avg,sum,min,max 等在求值的时候会忽略null值，但是count(*) 不会忽略null 值。先造出一个空值来count(cloumn_name) 也是会忽略null 值的。
SQL> update employees
2 set salary = null
3 where employee_id = 100;
1 row updated.
SQL> select count(*) as row_count,count(salary),
2 avg(salary) as avg_salary,sum(salary) as sum_salary,
3 min(salary) as min_salary,max(salary) as max_salary
4 from employees;
ROW_COUNT COUNT(SALARY) AVG_SALARY SUM_SALARY MIN_SALARY MAX_SALARY
---------- ------------- ---------- ---------- ---------- ----------
107 106 6296.22642 667400 2100 17000
三、在where 子句中使用null值。在where 中不能使用等于号指定null 值条件。应该使用is null或者is not null.下面查询salary 值为null 的员工的姓名。
SQL> select last_name,first_name
2 from employees
3 where salary = null;
no rows selected
SQL> select last_name,first_name
2 from employees
3 where salary is null;
LAST_NAME FIRST_NAME
------------------------- --------------------
King Steven
四、group by 子句中的null 值。先创建一个测试用的表testing02.
在group by 子句中不会忽略null 值，也会对null 进行分组。
SQL> create table testing02 (id number(6),name varchar2(20));
Table created.
SQL> insert into testing02 values(100,'testing');
1 row created.
SQL> insert into testing02 values(101,null);
1 row created.
SQL> commit;
Commit complet
SQL> select count(*) as row_count,name
2 from testing02
3 group by name;
ROW_COUNT NAME
---------- --------------------
1
1 testing
五、在order by 中的null 值。通过在order by 中指定nulls first,nulls last 来实现让带有null 值的列排在前面或者后面，不要太依赖select 语句查询的默认输出。
SQL> select * from testing02;
ID NAME
---------- --------------------
100 testing
101
SQL> select * from testing02
2 order by name nulls first;
ID NAME
---------- --------------------
101
100 testing
SQL> select * from testing02
2 order by name nulls last;
ID NAME
---------- --------------------
100 testing
101
六、在聚合运算union,union all，minus,intersect 中不会忽略null 值null 值将作为相等的值来对待此时可以认为null 等于 null.
7、null 值的插入。往表中的某一列插入一个null 值可以使用很多的方法，很多时候这取决于习惯。
SQL> insert into testing02 values(102,'');
1 row created.
SQL> insert into testing02 values(103,null);
1 row created.
SQL> insert into testing02 (id) values(103);
1 row created.
要注意的是下面这条语句不会插入null 值，而是插入字符串null.
SQL> insert into testing02 values(102,'null');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from testing02
2 where id > 101;
ID NAME
---------- --------------------
102
103
103
102 null