如何构建此关系数据库

Question

Basically I'll be having 3 tables that have relation. They are: users , departments and company .

The issue I have is this:

• A company can have many departments
• A department can only be attached to one company
• A user can only be part of one company
• A user can be part of many departments

This is essentially what the table relation would look like:

                    ____________________
                    | | | |            |
                    | | | |            |
--------      --------------      -----------
| user |      | department |      | company |
--------      --------------      -----------
 |   |         | | | | |               |
 |   |         | | | | |               |
 |   ___________________               |
 |                                     |
 |                                     |
 |                                     |
 _______________________________________

^{The above multiple | lines show an option, so the "company" above has 4 departments and so on.}

Now my question is this, How should I structure the relation tables?

Should I have user_departments , user_company and company_departments tables?

That would essentially look like this:

--------------------
| user_departments |
--------------------------------
| id | user_id | department_id |
--------------------------------

----------------
| user_company |
-----------------------------
| id | user_id | company_id |
-----------------------------

-----------------------
| company_departments |
-----------------------------------
| id | company_id | department_id |
-----------------------------------

Or are there any other alternatives for me to consider/implement instead of the path I'm going as it seems it'll just keep growing complex?

Answer 1

You are essentially making redundant relationships. You should have no need for a company_departments, company_id will just be a field of the departments table to reference the company a department is a part of. Likewise, you won't need a user_company table, but you will need the user_departments one; that is due to the user-department relationship actually being many-to-many.

With the example you've given, you should only need four tables.

company: company_id, other company info (such as name) 
department: department_id, company_id (referencing the company record), other department info 
user: user_id, company_id (referencing the company record), other user info
user_departments: user_id, department_id, perhaps information such as user's role in department, or if you want historical data preserved dates assigned to and removed from department

Answer 2

Here is the layout in the format you used:

---------
| users |
--------------------------------
| id | name | company_id | ... |
--------------------------------

-----------
| company |
-------------------
| id | name | ... |
-------------------

-----------
| departments |
--------------------------------
| id | name | company_id | ... |
--------------------------------

--------------------
| user_departments |
--------------------------------
| id | user_id | department_id |
--------------------------------