Re: Functional Dependency to constrain a relation to exactly one element?

Marshall wrote:
> It's easy enough to describe a functional dependency that will
> constrain
> a relation to having at most one row; it is any functional dependency
> for which the determinant set is empty.

Yep.

> Is it possible to have a functional dependency that will constrain
> R to *exactly* one row?

No. The empty relation satisfies all FDs.

> If it can't be done with an FD, can we do it with a constraint?

Of course. In first-order logic you can specify this (i.e. that there is at least one tuple in the relation) with a simple existential statement.

• Jan Hidders