组函数不计算null值

SELECT SUM(a) FROM (

SELECT 1 a FROM dual

UNION

SELECT 2 a FROM dual

UNION

SELECT 3 a FROM dual

);

结果是6

SELECT SUM(a) FROM (

SELECT 1 a FROM dual

UNION

SELECT 2 a FROM dual

UNION

SELECT 3 a FROM dual

UNION

SELECT NULL a FROM dual

);

结果还是6

例如：http://blog.itpub.net/23650854/viewspace-684335

select rownum,a from t_null;

ROWNUM          A---------- ----------         1         2         3

z@test10gr2> select sum(a) from t_null;

SUM(A)----------

NULL

select rownum,a from t_null;

ROWNUM A

---------- ----------

1

2

3

4 77

select sum(a) from t_null;

SUM(A)

----------

77

http://edgenhuang.iteye.com/blog/975567

-- oracle 将 空字符串即''当成null，测试脚本如下：

select nvl(null,-1) from dual;

select nvl('','-1') from dual;

select nvl(nvl(null,''),-1) from dual;

select nvl(trim('  '),'-1') from dual;

-- 但是要记住，null 与任何值做逻辑运算得结果都为 false，包括和null本身：

select nvl(max('1'),-1) from dual where null = '';

select nvl(max('1'),-1) from dual where null <> '';

select nvl(max('1'),-1) from dual where null = '-1';

select nvl(max('1'),-1) from dual where null <> '-1';

select nvl(max('1'),-1) from dual where null = null;

select nvl(max('1'),-1) from dual where null <> null;

-- 不过，用 is null 判断时，空字符串和 null 都 is null：

select nvl(max('1'),-1) from dual where '' is null;

select nvl(max('1'),-1) from dual where null is null;

--------------------------------------------------------------------------------------------------

-- 还要记住，null 和任何数值进行数学运算，结果都为 null：

select nvl(null + 0,-1) from dual;

-- 不过，可以使用 || 将空字符串或 null 和字符串连接

select 'a' || null || 'b' from dual;

--------------------------------------------------------------------------------------------------

-- 另，作为以下的 Oracle 内置的函数的参数时，结果也为 null：

select nvl(length(null),-1) from dual;

select nvl(trim(null),-1) from dual;

select nvl(ltrim(null),'-1') from dual;

select nvl(rtrim(null),'-1') from dual;

select nvl(rtrim(null,' '),'-1') from dual;

select nvl(soundex(null),'-1') from dual;

select nvl(SubStr(null,1),-1) from dual;

select nvl(InStr(null,1),-1) from dual;

select nvl(replace(null,'a','b'),'-1') from dual;

select nvl(min(null),'-1') from dual;

select nvl(max(null),'-1') from dual;

select nvl(sum(null),'-1') from dual;

select nvl(avg(null),'-1') from dual;

select nvl(sum(null),'-1') from dual;

-- 不过，作为以下的 Oracle 内置的函数的参数时，结果不为null：

select concat('a', null) from dual;

select concat(null, 'a') from dual;

select count(null) from dual;

-- 其他函数用的时候，也可以使用以上的方法测试