ordering error in query?

I am trying to keep my select statements compatible for running on
oracle or postgres. I ran into a problem with a query that is part of
a union. The original query clause is of the form:

select distinct f.id, '' from foo f;

I need the empty string as the second attribute because the first part
of the union has a second attribute. This works on oracle, but
postgres says:

ERROR: Unable to identify an ordering operator '<' for type 'unknown'
Use an explicit ordering operator or modify the query

If I get rid of 'distinct' postgres responds, but I don't get the
result I want. If I type

select distinct s.ben, cast ('' as varchar) from sitealertsview s;

I get the result I want, but it's not compatible with oracle.

Suggestions?

Thanks,

Sarah
smarie(at)ekno(dot)com

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