help writing a constraint

I have a table with a pair of columns that shouldn't both have values:

CREATE TABLE Foo (
    id SERIAL not null,
    hasBar integer default null REFERERENCES Bar,
    hasBaz integer default null REFERERENCES Baz,
    primary key (id)
);

I need to constrain this so that for any given row, it either hasBar or
hasBaz, or neither, but not both. I'm not entirely clear on how to write
constraints, though. Is the following correct (within the CREATE)?

CHECK (hasBar IS NULL OR hasBaz IS NULL)

--Greg