How to Create a real one-to-one relationship in SQL Server

I'm pretty sure it is technically impossible in SQL Server to have a True 1 to 1 relationship, as that would mean you would have to insert both records at the same time (otherwise you'd get a constraint error on insert), in both tables, with both tables having a foreign key relationship to each other.

That being said, your database design described with a foreign key is a 1 to 0..1 relationship. There is no constraint possible that would require a record in tableB. You can have a pseudo-relationship with a trigger that creates the record in tableB.

So there are a few pseudo-solutions

First, store all the data in a single table. Then you'll have no issues in EF.

Or Secondly, your entity must be smart enough to not allow an insert unless it has an associated record.

Or thirdly, and most likely, you have a problem you are trying to solve, and you are asking us why your solution doesn't work instead of the actual problem you are trying to solve (an XY Problem).

UPDATE

To explain in REALITY how 1 to 1 relationships don't work, I'll use the analogy of the Chicken or the egg dilemma. I don't intend to solve this dilemma, but if you were to have a constraint that says in order to add a an Egg to the Egg table, the relationship of the Chicken must exist, and the chicken must exist in the table, then you couldn't add an Egg to the Egg table. The opposite is also true. You cannot add a Chicken to the Chicken table without both the relationship to the Egg and the Egg existing in the Egg table. Thus no records can be every made, in a database without breaking one of the rules/constraints.

Database nomenclature of a one-to-one relationship is misleading. All relationships I've seen (there-fore my experience) would be more descriptive as one-to-(zero or one) relationships.

UPDATE EF 5.0 - one-to-one Support

While SQL Server will still allow the dependent row to be null. Entity Framework Core 5.0 now allows you to configure dependent properties as required. EF 5 What's new

Excerpt:

In EF Core 5.0, a navigation to an owned entity can be configured as a required dependent. For example:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Person>(b =>
    {
        b.OwnsOne(e => e.HomeAddress,
            b =>
            {
                b.Property(e => e.City).IsRequired();
                b.Property(e => e.Postcode).IsRequired();
            });
        b.Navigation(e => e.HomeAddress).IsRequired();
    });
}

Set the foreign key as a primary key, and then set the relationship on both primary key fields. That's it! You should see a key sign on both ends of the relationship line. This represents a one to one.

Check this : SQL Server Database Design with a One To One Relationship

This can be done by creating a simple primary foreign key relationship and setting the foreign key column to unique in the following manner:

CREATE TABLE [Employee] (
    [ID]    INT PRIMARY KEY
,   [Name]  VARCHAR(50)
);

CREATE TABLE [Salary] (
    [EmployeeID]    INT UNIQUE NOT NULL
,   [SalaryAmount]  INT 
);

ALTER TABLE [Salary]
ADD CONSTRAINT FK_Salary_Employee FOREIGN KEY([EmployeeID]) 
    REFERENCES [Employee]([ID]);

INSERT INTO [Employee] (
    [ID]
,   [Name]
)
VALUES
    (1, 'Ram')
,   (2, 'Rahim')
,   (3, 'Pankaj')
,   (4, 'Mohan');

INSERT INTO [Salary] (
    [EmployeeID]
,   [SalaryAmount]
)
VALUES
    (1, 2000)
,   (2, 3000)
,   (3, 2500)
,   (4, 3000);

Check to see if everything is fine

SELECT * FROM [Employee];
SELECT * FROM [Salary];

Now Generally in Primary Foreign Relationship (One to many), you could enter multiple times EmployeeID, but here an error will be thrown

INSERT INTO [Salary] (
    [EmployeeID]
,   [SalaryAmount]
)
VALUES
    (1, 3000);

The above statement will show error as

Violation of UNIQUE KEY constraint 'UQ__Salary__7AD04FF0C044141D'. Cannot insert duplicate key in object 'dbo.Salary'. The duplicate key value is (1).