Defining a one-to-one relationship in SQL Server
One-to-one is actually frequently used in super-type/subtype relationship. In the child table, the primary key also serves as the foreign key to the parent table. Here is an example:
CREATE TABLE Organization( ID int PRIMARY KEY, Name varchar(200), Address varchar(200), Phone varchar(12))GOCREATE TABLE Customer( ID int PRIMARY KEY, AccountManager varchar(100))GOALTER TABLE Customer ADD FOREIGN KEY (ID) REFERENCES Organization(ID) ON DELETE CASCADE ON UPDATE CASCADEGO
there is no such thing as an explicit one-to-one relationship.
But, by the fact that tbl1.id and tbl2.id are primary keys and tbl2.id is a foreign key referenceing tbl1.id, you have created an implicit 1:0..1 relationship.