空字符串与NULL
首先有如下代码
SELECT *
FROM Pdc_DataDomain DD
INNER JOIN Pdc_DD_Table DDT
ON DD.DataDomainID = DDT.DataDomainID
AND DD.ApplicationDBID = ‘3e7c6764d73f4c7786c99e5b72eb6912‘
AND DDT.TableName <> ‘#‘
AND NOT EXISTS
(SELECT *
FROM pdc_Application_3e7c6764 A
WHERE A.TableName = DDT.TableName
AND A.FieldName = DDT.FieldName
AND A.FieldType = DDT.FieldType
AND nvl(A.FieldLength, ‘‘) = nvl(DDT.FieldLength, ‘‘)
AND nvl(A.FieldScale, ‘‘) = nvl(DDT.FieldScale, ‘‘))
核心重点在这句AND nvl(A.FieldScale, ‘‘) = nvl(DDT.FieldScale, ‘‘)
上,这句当A的FieldScale
和DDT的FieldScale
都为空的时候,会出现两者nvl后都返回‘‘,但是Oracle的特性是‘‘会被处理为null,而null不等于任何值,包括自己,,那么就over了,如果出现上面这种应该如何处理呢,很简单,转化的时候可以将其转化为一个程序中不会出现的值即可,如果程序逻辑没有不会出现的值,就只有使用isnull
来判断了.
以下是参考资料:
-- oracle 将 空字符串即‘‘当成null,测试脚本如下:
select nvl(null,-1) from dual; sele‘‘,‘-1‘) from dual; select nvl(nvl(null,‘‘),-1) from dual; select nvl(trim(‘ ‘),‘-1‘) from dual;ct nvl(
-- 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
Oracle坑之-空字符串与NULL
时间: 2024-10-12 08:21:18