Question

I have 2 tables :

first one is something like that :

Person
       first_name (primary key)
       family_name (primary key)
       age
       [other things..]

second one is :

Doctor
       first_name
       family_name
       specialty

So basically, in the table Person, their can’t be 2 Person with the same first_name and family_name. This works well if I both set them on primary key using phpmyadmin.

My problem is that now I want to make it impossible to add a Doctor who is not a Person. I’ve tried putting a foreign key constraint on doctor.first_name and doctor.family_name, but it obviously doesn’t fix my issue.

(Don’t ask me to use an id or something, my tables are much more complicated than that and I can’t use an id, it has to be a table with composite primary key).

Answer 1

Your wording is not accurate. A table cannot have 2 Primary Keys. It can have, however, a compound (composite) Primary Key, that is made of 2 or more columns.

CREATE TABLE Person
(      first_name 
,      family_name 
,      age
,      ...
,   PRIMARY KEY (first_name, family_name)
)

Leaving aside discussions about surrogate vs. natural keys, when you have such a compound Primary Key, any Foreign Key from another table should be a compound one and reference your compound Primary Key:

CREATE TABLE Doctor
(      first_name
,      family_name
,      specialty
,      ...
,   PRIMARY KEY (first_name, family_name)

,   FOREIGN KEY (first_name, family_name)
      REFERENCES Person (first_name, family_name)
)  

This kind of foreign key constraint, where the PRIMARY KEY of a table (Doctor) is also a FOREIGN KEY to another table (Person) is the common solution for a 1::0..1 relationship (also called supertype/subtype).