constraint -- one or the other column not null

I have two columns, both individually nullable, but a row needs to have
a value in one or the other. What is the best way to implement the
constraints? I currently have:

create table f (
a int,
b int,
check (a + b is null),
check (coalesce(a,b) is not null)
);

Is there a better way to do it?

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George Pavlov
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