MySQL foreign key constraints through more tables - mysql

Question

I find myself stack to this problem. I've got the following 3 tables, which I can't modify (it'd be so nice):
Person
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| SSN | varchar(50) | NO | PRI | NULL | |
| name | varchar(50) | YES | UNI | NULL | |
| birthday | date | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
Employee
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| SSN | varchar(50) | NO | PRI | NULL | |
| department | varchar(50) | YES | | NULL | |
| salary | varchar(50) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
Employer
+------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| name | varchar(50) | NO | PRI | NULL | |
| department | varchar(50) | YES | | NULL | |
| salary | varchar(50) | YES | | NULL | |
+------------+-------------+------+-----+---------+-------+
Contract
+----------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| employer_name | varchar(50) | NO | PRI | NULL | |
| employee_name | varchar(50) | YES | PRI | NULL | |
+----------------+-------------+------+-----+---------+-------+
I know that:
employee(sin) is foreign key to person(sin),
employer(name) is foreign key to person(name),
contract(employer_name) is a foreign key to employer(name)
And I should insert another foreign key for contract(employee_name), though employee table has only the SSN. Is there a way to reference the foreign key to person.name, passing through employee table, something like
CONSTRAINTS FOREIGN KEY contract(employee_name) REFERENCES TO person(name) WHERE person(sin)=employee(sin);
?
Thank you very much for any help!

Answer 1

An SQL FK constraint says its referencing column list subrow values, if all non-NULL, have to appear as subrow values for its referenced column list, which must be declared UNIQUE/PK in a base table.
Your constraint is not an SQL FK constraint.
If we could use a query in the place of the referenced base table name in an SQL FK constraint then the constraint you would want would be:
Contract(person_name) REFERENCES
(SELECT name FROM Person p JOIN Employee e ON p.name = e.name)(name)
(It can be shown that name is UNIQUE in that table.) But we can't.
If MySQL supported CREATE ASSERTION then you could CHECK that every Contract person_name was IN (SELECT name FROM Person p JOIN Employee e ON p.name = e.name). But it doesn't.
So this is an example of a constraint that, if you can't redesign your tables, you would enforce by appropriate triggers when the tables involved change.
Is there a way to reference the foreign key to person.name, passing through employee table, something like
If you want to figure out situations like this then you have to forego the cop-outs of using precise terms like "foreign key" just because you are somehow reminded of the things they refer to but that aren't there, or using the poetic "passing through", or using the vague "something like". You have to actually write out the conditions that you want your tables to satisfy.