ingnoring range query if field is null or empty

Hi,
   I wanted to make use of a range query to find out data
within the range.
eg. minexp : 3 maxexp: 7 if a persons exp: is between 3 and
7 then I can get
his details.
The problem is that if in the index the minexp or the maxexp
is null/ empty
i don't want to use the range query for the field which is
null.
that means i want to make range query only for less if
maxexp is null.
how can i handle this situation???
thanks in advance

Prabin