如何通过多个表列主键引用一列

Question

I have a database consist of 3 tables each table indicate one user like this:

1. Admin table 'User 1'.

2. Staff table 'User 2'.

3. Student table 'user 3'.

I want to implement a common table between them called 'POST' WHERE each user can post content to the post table content column, however I need to Identify the type of posters ' the one who posted the post ' so I'm adding a column user_type to the table POST the column user_type should hold the ID of the USER ' Admin, Staff, Student '

I'm in a case where I want to be able to refer to this column ( user_type) in the POST table to multiple table columns Admin table, Staff table, Student table. So I Can identify the USER who posted the post.

Answer 1

All these kind of users are persons that you need to store in one parent table called "Users" or "Persons" from which the parent's table are "User 1", "User 2", "User 3"...

This is called inheritance in data modeling.

Thus you can join the post table to the father table.

As an example:

CREATE TABLE T_PERSON_PRS           (PRS_ID INT IDENTITY PRIMARY KEY, PRS_NAME...)
CREATE TABLE T_PERSON_ADMIN_PAM     (PRS_ID INT PRIMARY KEY FOREIGN KEY T_PERSON_PRS (PRS_ID), ... )
CREATE TABLE T_PERSON_STAFF_PSF     (PRS_ID INT PRIMARY KEY FOREIGN KEY T_PERSON_PRS (PRS_ID), ... )
CREATE TABLE T_PERSON_STUDENT_PSD   (PRS_ID INT PRIMARY KEY FOREIGN KEY T_PERSON_PRS (PRS_ID), ... )
CREATE TABLE T_POST_PST             (PST_ID INT IDENTITY PRIMARY KEY, PRS_ID INT PRIMARY KEY FOREIGN KEY T_PERSON_PRS (PRS_ID), ...)