Question

I’m putting together my own database and from examples I’ve seen, Foriegn Key can also be set as Primary Keys.

I was creating my Tables so that all of my FK were also PK. Is this wrong? When should a FK be a PK? Does it have to be a PK?

Primary Key’s make sense in their own table… as the Id and Identity. But when using the Id is another table, does it have to be a PK as well?

Answer 1

A Foreign Key should only be the Primary Key when your trying to create a 1 to 1 or 1 to zero/1 mapping.

Example:

I have a Person table, an Employee table, and a Contractor table. All Employees are people, all Contractors are people and every Person is either an employee or a Contractor

Essentially you would end up with something like this.

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In response to your people have multiple addresses you should create an association table. Here is a diagram.

As you can see now every person can have many addresses and since each Employee is a person then every Employee can have many addresses. This is the same for Contractor as well.

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Edited: Here is the Change Script from SQL Server

BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Address
    (
    AddressId bigint NOT NULL,
    Address nvarchar(50) NULL,
    City nvarchar(50) NULL,
    State nvarchar(50) NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.Address ADD CONSTRAINT
    PK_Address PRIMARY KEY CLUSTERED 
    (
    AddressId
    ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

GO
ALTER TABLE dbo.Address SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Person
    (
    PersonId bigint NOT NULL,
    Name nvarchar(50) NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.Person ADD CONSTRAINT
    PK_Person PRIMARY KEY CLUSTERED 
    (
    PersonId
    ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

GO
ALTER TABLE dbo.Person SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.PersonAddress
    (
    PersonId bigint NOT NULL,
    AddressId bigint NOT NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.PersonAddress ADD CONSTRAINT
    PK_PersonAddress PRIMARY KEY CLUSTERED 
    (
    PersonId,
    AddressId
    ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

GO
ALTER TABLE dbo.PersonAddress ADD CONSTRAINT
    FK_PersonAddress_Person FOREIGN KEY
    (
    PersonId
    ) REFERENCES dbo.Person
    (
    PersonId
    ) ON UPDATE  NO ACTION 
     ON DELETE  NO ACTION 

GO
ALTER TABLE dbo.PersonAddress ADD CONSTRAINT
    FK_PersonAddress_Address FOREIGN KEY
    (
    AddressId
    ) REFERENCES dbo.Address
    (
    AddressId
    ) ON UPDATE  NO ACTION 
     ON DELETE  NO ACTION 

GO
ALTER TABLE dbo.PersonAddress SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Employee
    (
    EmployeeId bigint NOT NULL,
    EmployeeNumber nvarchar(50) NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.Employee ADD CONSTRAINT
    PK_Employee PRIMARY KEY CLUSTERED 
    (
    EmployeeId
    ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

GO
ALTER TABLE dbo.Employee ADD CONSTRAINT
    FK_Employee_Person FOREIGN KEY
    (
    EmployeeId
    ) REFERENCES dbo.Person
    (
    PersonId
    ) ON UPDATE  NO ACTION 
     ON DELETE  NO ACTION 

GO
ALTER TABLE dbo.Employee SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Contractor
    (
    ContractorId bigint NOT NULL,
    ContractorNumber nvarchar(50) NULL
    )  ON [PRIMARY]
GO
ALTER TABLE dbo.Contractor ADD CONSTRAINT
    PK_Contractor PRIMARY KEY CLUSTERED 
    (
    ContractorId
    ) WITH( STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]

GO
ALTER TABLE dbo.Contractor ADD CONSTRAINT
    FK_Contractor_Person FOREIGN KEY
    (
    ContractorId
    ) REFERENCES dbo.Person
    (
    PersonId
    ) ON UPDATE  NO ACTION 
     ON DELETE  NO ACTION 

GO
ALTER TABLE dbo.Contractor SET (LOCK_ESCALATION = TABLE)
GO
COMMIT