A pharmacy chain has hired you to design its database, and you have gathered the following information:

Patients are identified by an SSN$,$ and we store their names, addresses, and ages.

Doctors are also identified by an SSN$.$ For each doctor, we also record name, specialty, and years of experience.

For each drug, the trade name $($unique$)$ and formula must be stored. We also store its cost, color, size, and shape.

Every patient has a primary doctor and every doctor has at least one patient.

Doctors prescribe drugs to patients. Each prescription has a date and a quantity associated with it$.$ If a doctor prescribes the same drug for the same patient more than once, only the most recent of those prescriptions needs to be stored.

Some drugs can be substituted for other drugs, and we want to store that information in the database also. A drug may have many substitutes, and a drug could substitute for any number of other drugs.