T-SQL foreign key check constraint

When you create a foreign key constraint in a table and you create the script in MS SQL Management Studio, it looks like this.

ALTER TABLE T1  WITH CHECK ADD  CONSTRAINT FK_T1 FOREIGN KEY(project_id)
REFERENCES T2 (project_id)
GO
ALTER TABLE T1 CHECK CONSTRAINT FK_T1
GO

What I don't understand is what purpose has the second alter with check constraint. Isn't creating the FK constraint enough? Do you have to add the check constraint to assure reference integrity ?

Another question: how would it look like t开发者_运维百科hen when you'd write it directly in the column definition?

CREATE TABLE T1 (
my_column INT NOT NULL CONSTRAINT FK_T1 REFERENCES T2(my_column)
)

Isn't this enough?

First it creates the constraint and here you can specify whether data allready in the table should be checked or not against your new constraint. WITH { CHECK | NOCHECK }

The second part specifies that the constraint is enabled. ALTER TABLE TableName { CHECK | NOCHECK } CONSTRAINT ConstraintName

The second statement is compelled by the "WITH CHECK" in the first statement. There is a setting you can toggle to not do this.