oracle sum null值相关 null 和 空字符串('')

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