sql - 在 ORACLE SQL 的 where 子句中对空字符串使用修剪

Question

我试图弄清楚为什么下面的 SQL 语句不返回 oracle 中的值

Select 'do' from dual
where trim('  ') = ''

然而

Select 'do' from dual
where trim(' a ')='a'

返回值 do。

Answer 1

因为 trim(' ')返回 null而不是 ''
SQLFiddle example

In Oracle 8, there is no such thing as a zero-length string. Any zero-length string, either from a function call or the literal '', is treated as null.

Source