Question

I have a relational database with a Client table, containing id, name, and address, with many phone numbers
and I have an Employee table, also containing id, name, address, etc., and also with many phone numbers.

Is it more logical to create one “Phone Number” table and link the Clients and Employees, or to create two separate “Phone Number” tables, one for Clients and one for Employees?

If I choose to create one table, can I use one foreign key for both the Client and Employee or do I have to make two foreign keys?

If I choose to make one foreign key, will I have to make the Client ids start at 1 and increment by 5, and Employee ids start at 2 and increment by 5 so the two ids will not be the same?

If I create two foreign keys will one have a value and the other allow nulls?

Answer 1

The solution which I would go with would be:

CREATE TABLE Employees (
    employee_id    INT            NOT NULL,
    first_name     VARCHAR(30)    NOT NULL,
    ...
    CONSTRAINT PK_Employees PRIMARY KEY (employee_id)
)

CREATE TABLE Customers (
    customer_id    INT            NOT NULL,
    customer_name  VARCHAR(50)    NOT NULL,
    ...
    CONSTRAINT PK_Customers PRIMARY KEY (customer_id)
)

-- This is basic, only supports U.S. numbers, and would need to be changed to
-- support international phone numbers
CREATE TABLE Phone_Numbers (
    phone_number_id    INT            NOT NULL,
    area_code          CHAR(3)        NOT NULL,
    prefix             CHAR(3)        NOT NULL,
    line_number        CHAR(4)        NOT NULL,
    extension          VARCHAR(10)    NULL,
    CONSTRAINT PK_Phone_Numbers PRIMARY KEY (phone_number_id),
    CONSTRAINT UI_Phone_Numbers UNIQUE (area_code, prefix, line_number, extension)
)

CREATE TABLE Employee_Phone_Numbers (
    employee_id        INT    NOT NULL,
    phone_number_id    INT    NOT NULL,
    CONSTRAINT PK_Employee_Phone_Numbers PRIMARY KEY (employee_id, phone_number_id)
)

CREATE TABLE Customer_Phone_Numbers (
    customer_id        INT    NOT NULL,
    phone_number_id    INT    NOT NULL,
    CONSTRAINT PK_Customer_Phone_Numbers PRIMARY KEY (customer_id, phone_number_id)
)

Of course, the model might changed based on a lot of different things. Can an employee also be a customer? If two employees share a phone number how will you handle it on the front end when the phone number for one employee is changed? Will it change the number for the other employee as well? Warn the user and ask what they want to do?

Those last few questions don’t necessarily affect how the data is ultimately modeled, but will certainly affect how the front-end is coded and what kind of stored procedures you might need to support it.