A doctor may refer a patient to other doctors for treatment. A doctor must be referred by at least one other doctors.Each referral captures the referral date and description. Each referral is only for one patient. A patient may have any number of referrals.
(you must reflect which doctor refers a specific patient to another doctor in your data model.)
How to draw the relationship?
is it unary or tertiary relation?
by the way...if the supertype has just one subtype, how to draw on the ER diagram?