How do I query on null or blank field?

[Frank]

Hello,

Using the ExpressionFactory, I am trying to locate records based on this sql:

WHERE TYPE_ADMIT = '{s_admittype}' AND
(active = 1) AND (LEN(LTRIM(ASSIGNED_ROOM_BED)) = 0) OR 
TYPE_ADMIT = '{s_admittype}' AND (active = 1) AND (ASSIGNED_ROOM_BED IS NULL)

Is ExpressionFactory the best way to do this?
I cannot seem to limit my records based on a field that can be blank or null.

Thanks for the help.

Frank