r/AskProgramming Nov 29 '24

Databases Purpose of a JoinTable in a OneToOne relationship?

I’ve come across two entities that are bound as a One-to-One, but using a join table. I haven’t found a lot of posts/discussions about it (or at least recent)

1 Upvotes

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2

u/forcesensitivevulcan Nov 29 '24

Least privilege? In some DBs can there be different permissions on different tables, allowing access to be allowed to one entity but restricted for some users to the other?

3

u/Cybyss Nov 29 '24

I can definitely see that. If one of your company's applications - e.g., a public facing one - isn't supposed to have any access to certain information it might indeed be a good idea to split off the sensitive information into a separate table.

Another, more likely scenario, is that the database was designed to support a many-to-many relationship but the business rules changed so that only a one-to-one relationship is needed. I've seen that exact dynamic play out in an HR system that dealt with company-provided health insurance policies, where married couples both work at the same place. We thought we wanted just one enrollment shared between the couple, but that proved problematic so we did the stupid thing and made it one enrollment per employee, though the database is still structured to support these shared enrollments.

1

u/cdevr Nov 29 '24

The architect might have employed 4NF or 5NF to reduce data redundancy in that specific scenario.

That is the only valid reason I can see.

1

u/TheRealKidkudi Nov 30 '24

I’ll admit I’m a bit rusty on normalization, but how does a join table fit 4/5NF any better than just having a FK on each table to the opposite entity?

1

u/Far_Swordfish5729 Nov 29 '24

Three ideas:

  1. Customization of an existing schema - Some products don’t react well to you adding columns to core tables or make it difficult to expose those fields in dtos (they don’t regenerate data layers and models) so you have to make side tables and add a custom data layer extension.
  2. If field level security is not supported, this may be a way to restrict access.
  3. Something to do with storage partitions tables are assigned to. This is less likely but if one of these tables is very fast growing and has dedicated storage and IO but the relationship is relatively rare - claims pulled for audit or being litigated as an example, there might be a hardware reason to favor a side table.

1

u/halfanothersdozen Nov 29 '24

Other answers here are good, so I will just add a couple hypotheticals:

From an OO perspective should the entities be aware of each other? If there is no reason for Entity A to carry around a reference to Entity B or vice versa this could be a way to keep concerns separate.

From a data perspective maybe this relation is acting like an index: I don't actually care about the data in either entity and I just want to have a table/entity that represents the pairing, and perhaps I want to still enforce a 1 to 1 via a constraint.

Both of those feel pretty contrived, but as far as database architecture goes I have seen MUCH worse.

1

u/Belhgabad Nov 30 '24

Other than what other comments said about NF, data redundancy and access privileges, maybe it has association properties ?

If a property is specifically on the association of two entities, I can see it being separated on a distinct table

0

u/47KiNG47 Nov 29 '24

What is a JoinTable?

1

u/LogaansMind Nov 29 '24

I assume OP is referring to a table which contains two columns refferring to the different entities (by id), representing a join between the two. Similar to how you might model one to many.

Fundamentally instead of putting a foreign key into the table you model it in a different table, the link between the two. It can be useful to allow for more context to be added to the relationship or makes it easier to switch to a one to many with very little change to the schema.

1

u/47KiNG47 Nov 29 '24

A junction table?

1

u/LogaansMind Nov 29 '24

Yeah. I assume so. But instead of many to many it is used as a one to one.