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.

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).

There is one way I know how to achieve a strictly* one-to-one relationship without using triggers, computed columns, additional tables, or other 'exotic' tricks (only foreign keys and unique constraints), with one small caveat.

I will borrow the chicken-and-the-egg concept from the accepted answer to help me explain the caveat.

It is a fact that either a chicken or an egg must come first (in current DBs anyway). Luckily this solution does not get political and does not prescribe which has to come first - it leaves it up to the implementer.

The caveat is that the table which allows a record to 'come first' technically can have a record created without the corresponding record in the other table; however, in this solution, only one such record is allowed. When only one record is created (only chicken or egg), no more records can be added to any of the two tables until either the 'lonely' record is deleted or a matching record is created in the other table.

Solution:

Add foreign keys to each table, referencing the other, add unique constraints to each foreign key, and make one foreign key nullable, the other not nullable and also a primary key. For this to work, the unique constrain on the nullable column must only allow one null (this is the case in SQL Server, not sure about other databases).

CREATE TABLE dbo.Egg (
    ID int identity(1,1) not null,
    Chicken int null,
    CONSTRAINT [PK_Egg] PRIMARY KEY CLUSTERED ([ID] ASC) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE dbo.Chicken (
    Egg int not null,
    CONSTRAINT [PK_Chicken] PRIMARY KEY CLUSTERED ([Egg] ASC) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE dbo.Egg  WITH NOCHECK ADD  CONSTRAINT [FK_Egg_Chicken] FOREIGN KEY([Chicken]) REFERENCES [dbo].[Chicken] ([Egg])
GO
ALTER TABLE dbo.Chicken  WITH NOCHECK ADD  CONSTRAINT [FK_Chicken_Egg] FOREIGN KEY([Egg]) REFERENCES [dbo].[Egg] ([ID])
GO
ALTER TABLE dbo.Egg WITH NOCHECK ADD CONSTRAINT [UQ_Egg_Chicken] UNIQUE([Chicken])
GO
ALTER TABLE dbo.Chicken WITH NOCHECK ADD CONSTRAINT [UQ_Chicken_Egg] UNIQUE([Egg])
GO

To insert, first an egg must be inserted (with null for Chicken). Now, only a chicken can be inserted and it must reference the 'unclaimed' egg. Finally, the added egg can be updated and it must reference the 'unclaimed' chicken. At no point can two chickens be made to reference the same egg or vice-versa.

To delete, the same logic can be followed: update egg's Chicken to null, delete the newly 'unclaimed' chicken, delete the egg.

This solution also allows swapping easily. Interestingly, swapping might be the strongest argument for using such a solution, because it has a potential practical use. Normally, in most cases, a one-to-one relationship of two tables is better implemented by simply refactoring the two tables into one; however, in a potential scenario, the two tables may represent truly distinct entities, which require a strict one-to-one relationship, but need to frequently swap 'partners' or be re-arranged in general, while still maintaining the one-to-one relationship after re-arrangement. If the more common solution were used, all data columns of one of the entities would have to be updated/overwritten for all pairs being re-arranged, as opposed to this solution, where only one column of foreign keys need to be re-arranged (the nullable foreign key column).

Well, this is the best I could do using standard constraints (don't judge :) Maybe someone will find it useful.