Finding Composite Keys in Informix?

I have an undocumented database I am trying to reverse-engineer. In particular I want to find potential candidate keys. Typically I use the following syntax (for keys consisting of a single column): If select count(*) from t; = select count(distinct c) from t; then c is a potential candidate key for table t. To search for potential composite candidate keys I try: select count(distinct c1, c2) from t; This works with a number of other DBMS where the semantics of the query is to count the number of distinct (c1,c2) pairs. Informix, however, gives me a syntax error. When I omit the count aggregate, however, it works as expected. select distinct c1, c2 from t; Is there something wrong with the way I am trying to find potential composite candidate keys? Is there another way to do this in SQL? Thanks for any help, -- Robert Winkler U.S. Army Research Laboratory Beckman Institute, Room 4357 405 North Matthews Ave, Urbana, IL 61801