Put the whole where clause in the variable:

IF you want to filter records
lcWhere = 'WHERE
ELSE
lcWhere = ''
ENDIF

SELECT * ;
FROM table (lcWhere) INTO CURSOR cTemp

The string will either evaluate to blanks and you will not have a where clause which returns all records or whatever where clause you want which gives you only the filter condition. We do the same thing for joins and a lot of stuff in the SQL statement.