SQL query re unions and nulls

I'd really appreciate some help with this particular SQL problem I've been landed with. I need to write two selects joined by a union to deliver separate subsets of columns. Consider a table x with columns a,b,c,d,e all of which are type integer. Depending on various entries in the where clauses, I need columns a,b,c,d from the first select and a,b,c,e from the second but I need all five columns in total. What I need is something like the following: select a, b, c, d, null from x where .... union select a, b, c, null, e from x where ... The problem is how to specify a literal null - if the fields where char(n) then I could live with blanks or even two quotes together but that won't work for numeric type fields. One last thing, I need a solution that will work on Informix and Sybase through ODBC. Thanks in advance if anyone can help