Consider relation PHD with attributes P $($patient$),$ H $($hospital$),$ and D $($doctor$).$ The meaning of a PHD tuple

$($p$,$h$,$d$)$ is that patient p visited hospital h and has been examined by doctor d$.$ The following constraints

hold: $(6$ points$)$

$-$ In each hospital, each patient is examined by only one doctor.

$-$ Each doctor works for only one hospital.