Excluding null values on SQL table query

[DB2010MN26]

Tryed to search for this but couldnt find much on it. I'm creating a simple query off of an SQL table. I'm trying to filter out the blank values so naturally I tryed the "Is Not Null" command in the criteria for the field I wanted to filter. When I did this it would continue to return all the values in the table including the blank ones. I was able to succesfully filter them out with the command ' <>"" ' Does anyone know why this would work and not the "Is Not Null" command. Is it somethign to do with the fact that i'm accessing an SQL table?

[weekend00]

because you have empty strings ("" or " ") in the column instead of nulls