Question

Are Foreign Keys in SQL Server 2008 are automatically indexed with a value? For Example. if I add a value in my Primary key (or auto incremetend) in may parent table will the table that has a foreign key referenced to that key will automatically have the same value? or I Have to do it explicitly?

Answer 1

No, if you create a foreign key in a child table, it will not automatically get populated when a parent row gets inserted. If you think about this it makes sense. Let’s say you have a table like:

CREATE TABLE dbo.Students
(
  StudentID INT IDENTITY(1,1) PRIMARY KEY,
  Name SYSNAME
);

CREATE TABLE dbo.StudentLoans
(
  LoanID INT IDENTITY(1,1) PRIMARY KEY,
  StudentID INT FOREIGN KEY REFERENCES dbo.Students(StudentID),
  Amount BIGINT -- just being funny
);

What you are suggesting is that when you add a row to Students, the system should automatically add a row to StudentLoans – but what if that student doesn’t have a loan? If the student does have a loan, what should the amount be? Should the system pick a random number?

Typically what will happen in this scenario is that you’ll be adding a student and their loan at the same time. So if you know the loan amount and the student’s name, you can say:

DECLARE 
  @Name       SYSNAME = N'user962206', 
  @LoanAmount BIGINT = 50000,
  @StudentID  INT;

INSERT dbo.Students(Name) 
  SELECT @Name;

SELECT @StudentID = SCOPE_IDENTITY();

INSERT dbo.StudentLoans(StudentID, Amount)
  SELECT @StudentID, @LoanAmount;