两个不同的主键使用相同的外键

Question

my problem is that I don't know and I cannot figure out if it is possible to have two different primary keys refering the same foreighn key. To be more specific, I have three tables. The first table is called staff with attributes: id fname lname password . The second table is called student with attributes: id fname lname password role and finally the third table that is called message and it has 4 attributes: id text date user_fk and I want the two primary keys to refer the foreighn key. In phpmyadmin is quite easy and I made it. The problem is that when I want to enter a new row the database, I suppose, cannot figure out if the primary key is from the staff table or from the student. The message that I get is: #1452 - Cannot add or update a child row: a foreign key constraint fails ( diss . message , CONSTRAINT student_fk FOREIGN KEY ( user_fk ) REFERENCES student ( studentid )) . So my question is: is it completely wrong what I want to do? Maybe it will be better to create for example another table that will store only the foreighn keys? Or the error is somewhere else that I cannot find it? Thank you!

Answer 1

If I understand you correctly, the foreign key user_fk in message refers to student and staff ids.

In that case, if you try to insert data to message table, it will check both tables student and staff for that id. Hence the error.

What you can do is add two foreign key columns in message one for each table referred.

Update

The above approach will lead to the possibility of nullable foreign keys. We can instead have two mapping tables one for each table. for eg: student_message_map and staff_message_map . Each table will contain 2 foreign keys student_id and message_id / staff_id and message_id .

This will make the columns not nullable but querying will need to do two joins. Also, someone looking at this relationship may think it is many to many relationship because of the mapping tables.

IMO having a nullable foreign key is not bad practice, you just have to be aware while querying, about the possibility of null values.