When I should use one to one relationship?

Question

Sorry for that noob question but is there any real needs to use one-to-one relationship with tables in your database? You can implement all necessary fields inside one table

Answer 1

not very often.

you may find some benefit if you need to implement some security - so some users can see some of the columns (table1) but not others (table2)..

of course some databases (Oracle) allow you to do this kind of security in the same table, but some others may not.

Answer 2

As with all design questions the answer is "it depends."

There are few considerations:

• how large will the table get (both in terms of fields and rows)? It can be inconvenient to house your users' name, password with other less commonly used data both from a maintenance and programming perspective

• fields in the combined table which have constraints could become cumbersome to manage over time. for example, if a trigger needs to fire for a specific field, that's going to happen for every update to the table regardless of whether that field was affected.

• how certain are you that the relationship will be 1:1? As This question points out, things get can complicated quickly.

Answer 3

I normally encounter two general kinds of 1:1 relationship in practice:

1. IS-A relationships, also known as supertype/subtype relationships. This is when one kind of entity is actually a type of another entity (EntityA IS A EntityB). Examples:

   • Person entity, with separate entities for Accountant, Engineer, Salesperson, within the same company.
   • Item entity, with separate entities for Widget, RawMaterial, FinishedGood, etc.
   • Car entity, with separate entities for Truck, Sedan, etc.

   In all these situations, the supertype entity (e.g. Person, Item or Car) would have the attributes common to all subtypes, and the subtype entities would have attributes unique to each subtype. The primary key of the subtype would be the same as that of the supertype.

2. "Boss" relationships. This is when a person is the unique boss or manager or supervisor of an organizational unit (department, company, etc.). When there is only one boss allowed for an organizational unit, then there is a 1:1 relationship between the person entity that represents the boss and the organizational unit entity.

Answer 4

You are referring to database normalization. One example that I can think of in an application that I maintain is Items. The application allows the user to sell many different types of items (i.e. InventoryItems, NonInventoryItems, ServiceItems, etc...). While I could store all of the fields required by every item in one Items table, it is much easier to maintain to have a base Item table that contains fields common to all items and then separate tables for each item type (i.e. Inventory, NonInventory, etc..) which contain fields specific to only that item type. Then, the item table would have a foreign key to the specific item type that it represents. The relationship between the specific item tables and the base item table would be one-to-one.

Below, is an article on normalization.

http://support.microsoft.com/kb/283878

Answer 5

My 2 cents.

I work in a place where we all develop in a large application, and everything is a module. For example, we have a users table, and we have a module that adds facebook details for a user, another module that adds twitter details to a user. We could decide to unplug one of those modules and remove all its functionality from our application. In this case, every module adds their own table with 1:1 relationships to the global users table, like this:

create table users ( id int primary key, ...);
create table users_fbdata ( id int primary key, ..., constraint users foreighn key ...)
create table users_twdata ( id int primary key, ..., constraint users foreighn key ...)

Answer 6

In my time of programming i encountered this only in one situation. Which is when there is a 1-to-many and an 1-to-1 relationship between the same 2 entities ("Entity A" and "Entity B").

When "Entity A" has multiple "Entity B" and "Entity B" has only 1 "Entity A" and "Entity A" has only 1 current "Entity B" and "Entity B" has only 1 "Entity A".

For example, a Car can only have one current Driver, and the Driver can only drive one car at a time - so the relationship between the concepts of Car and Driver would be 1 to 1. - I borrowed this example from @Steve Fenton's answer

Where a Driver can drive multiple Cars, just not at the same time. So the Car and Driver entities are 1-to-many or many-to-many. But if we need to know who the current driver is, then we also need the 1-to-1 relation.