Select on a date and the return can not be null

Assign the IS NOT NULL as it will filter all non-NULL values.

SELECT * FROM Table WHERE DataInicio IS NOT NULL

Try this:

select case
         when campo_que_pode_estar_null is null then 'valor retornado se estiver null'
       else
         campo_que_pode_estar_null
       end 
from tabela

With this, even if the field is null, it will return something.

I forgot to mention that the database was Firebird.

But I found a solution, I used coalesce('nomedacoluna',0 ou'') as 'nomedacoluna'