🚀 go-pugleaf

2 questions:

How do I create a SQL statement to NOT select fields that are blank.  It
is a character field, and when I try select * from mdbf for mfield IS
NOT NULL, I still get rows that contain the blank field in my result
rowset.

Second question:

How do I achieve the equivalient of a unique index, to eliminate
duplicates in my rowset?  I have a unique index available, so do I
choose it as the active index after I have activated the query, or is
there a way to make the query SQL statement choose unique rows only?

I see the duplicate check box in the SQL query designer, but don't know
if this does it.

What happens if you activate a unique index after the query anyway? 
Does the index take precedence over the SQL statement, or simply act as
a subsequent filtering effect?