Using trim on empty string in where clause in ORACLE SQL

Question

I'm trying to figure out why the below SQL statement does not return the value do in oracle

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

Where as

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

returns the value do.

Answer 1

because trim(' ') returns null and not ''

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