Re: Multiple foreign keys

On Fri, 2003-01-24 at 08:25, Prokopis Prokopidis wrote:
> I was thinking of creating three foreign keys in the "comment" table,
> say "fk_student", "fk_teacher" and "fk_staff". Now, is there a way to
> impose a constraint that a comment row must have one and only one NOT
> NULL fk value?

Add this table constraint:
CONSTRAINT "only one not null"
CHECK ((fk_student IS NOT NULL AND fk_teacher IS NULL AND fk_student IS NULL)
OR (fk_student IS NULL AND fk_teacher IS NOT NULL AND fk_student IS NULL)
OR (fk_student IS NULL AND fk_teacher IS NULL AND fk_student IS NOT NULL)
)

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Oliver Elphick Oliver(dot)Elphick(at)lfix(dot)co(dot)uk
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