Question

I have an assignment where I am to create two tables within a database. The tables looks like this;

ContactPerson (ID, Forename, Surname, Email, PhoneNumber)
Company (ID, CompanyName)

Now my problem is that I have to link a ContactPerson to a specific company, but I can’t have them in the same table.

I understand that I can use the join statement to show both tables in one query but I need the database to know which person is linked to which company when I implement this databse into my asp.net project.

How do I do this?

Answer 1

You did say “specific” company so I’m assuming you have one company per person.

Put a column in the user table called CompanyID…

ALTER TABLE ContactPerson
ADD CompanyID int 

(assuming your ids are ints)
and then create the following foreign key:

ALTER TABLE [dbo].ContactPerson
    ADD CONSTRAINT [FK_ContactPerson_Company] 
    FOREIGN KEY (CompanyID)
    REFERENCES Company (ID) 

Shark is correct if you want a many to many relationship.

To get all people in a company:

SELECT
*
FROM
ContactPerson
WHERE
CompanyID = x

You don’t HAVE to apply the foreign key constraint, but if you don’t, you can accidentally put invalid data in. All a “Constraint” does is enforce a rule for you, in other words “making sure sql knows which people are in which company” as your question suggests you need to do.

The above query would work without the Foreign key constraint, but then your database doesn’t “know” about the relationship.

..and if I try and insert a person with a companyid that doesn’t exist, SQL will throw an error (this is a good thing).