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)
)
--
Oliver Elphick Oliver.Elphick@lfix.co.uk
Isle of Wight, UK http://www.lfix.co.uk/oliver
GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839 932A 614D 4C34 3E1D 0C1C
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"If anyone has material possessions and sees his
brother in need but has no pity on him, how can the
love of God be in him?"
I John 3:17
