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Asked: 2026-06-10T13:28:14+00:00

I’ve read several posts on SO about creating one-to-one relationship: how do i create

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I’ve read several posts on SO about creating one-to-one relationship:

how do i create a real one-to-one relationship in sql server

is there ever a time where using a database 11 relationship makes sense?

Database design 1 to 1 relationship

but I would be pleased to have your advice on this basic case: I have a USER and a COMPANY tables. Each USER can have 0 or 1 COMPANY.

Is it better to use two tables with the following relationship:

USER_COMPANY

RELATIONSHIP

or did I have just to use only one table with all fields needed:

USER

Thanks for your explanation.

Note: I’m using SQL Server (with Manager Studio to set it up) and plan to use EF.

Update: In order to be more explicit, what I’m trying to achieve is the following:
A user can own or doesn’t own a company. If he owns a company he is the sole person working for it.

Thanks

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1 Answer

  1. Editorial Team

    Since it is possible for a user not to own a company, this is not a true “1 to 1” relationship.

    In fact, this is “1 to 0..1”, and you can model it in one of the two ways:

    1. Have everything in one table:

      enter image description here

      Note how COMPANY_ID is both UNIQUE (preventing multiple users from owning the same company) and NULL-able (allowing for the users that don’t own a company). The separation of USER_ID and COMPANY_ID is what allows company-level foreign keys (i.e. allows child tables to reference company, while preventing them from referencing company-less users).

      If there are no company-level FKs, you can omit the COMPANY_ID altogether.

      You’ll also need a CHECK to ensure no other company field can be set unless COMPANY_ID is set (or at the very least that the correct subset of company fields is non-NULL).

    2. Have two tables:

      enter image description here

      We can’t just have PKs of these two tables also be FKs (in both directions) because MS SQL Server does’t support deferred constraints that would be needed to resolve the chicken-and-egg problem when inserting new data, nor it would correctly model the “1 to 0..1” relationship (it would model “1 to 1” and not allow company-less users).

    Which one of these two strategies should you choose depends largely on the number of companies compared to users:

    • If most users own a company, choose (1).
    • If there are many more users than companies, choose (2).
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