SQL one to one relationship vs. single table

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Consider a data structure such as the below where the user has a small number of fixed settings.

User
[Id] INT IDENTITY NOT NULL,
[Name] NVARCHAR(MAX) NOT NULL,
[Email] VNARCHAR(2034) NOT NULL
UserSettings
[SettingA],
[SettingB],
[SettingC]

Is it considered correct to move the user's settings into a separate table, thereby creating a one-to-one relationship with the users table? Does this offer any real advantage over storing it in the same row as the user (the obvious disadvantage being performance).

You would normally split tables into two or more 1:1 related tables when the table gets very wide (i.e. has many columns). It is hard for programmers to have to deal with tables with too many columns. For big companies such tables can easily have more than 100 columns.

So imagine a product table. There is a selling price and maybe another price which was used for calculation and estimation only. Wouldn't it be good to have two tables, one for the real values and one for the planning phase? So a programmer would never confuse the two prices. Or take logistic settings for the product. You want to insert into the products table, but with all these logistic attributes in it, do you need to set some of these? If it were two tables, you would insert into the product table, and another programmer responsible for logistics data would care about the logistic table. No more confusion.

Another thing with many-column tables is that a full table scan is of course slower for a table with 150 columns than for a table with just half of this or less.

A last point is access rights. With separate tables you can grant different rights on the product's main table and the product's logistic table.

So all in all, it is rather rare to see 1:1 relations, but they can give a clearer view on data and even help with performance issues and data access.

EDIT: I'm taking Mike Sherrill's advice and (hopefully) clarify the thing about normalization.

Normalization is mainly about avoiding redundancy and relateded lack of consistence. The decision whether to hold data in only one table or more 1:1 related tables has nothing to do with this. You can decide to split a user table in one table for personal information like first and last name and another for his school, graduation and job. Both tables would stay in the normal form as the original table, because there is no data more or less redundant than before. The only column used twice would be the user id, but this is not redundant, because it is needed in both tables to identify a record.

So asking "Is it considered correct to normalize the settings into a separate table?" is not a valid question, because you don't normalize anything by putting data into a 1:1 related separate table.

Why not combine all one-to-one relationship tables into a single table?, 6 years Oracle and 6 years SQL Server. Answered Jun 12, 2014 · Author has 404 answers and 625.2k answer views. This would be a three-table solution. One for users, one for all possible settings, and one for the junction table between them. The junction table can be quite useful. For instance, it might contain the effective and end dates of the setting. However, this assumes that the settings are "similar" to each other, in a SQL sense.

One-to-one relationships, In a one-to-one relationship, one record in a table is associated with one and only one record in another table. For example, in a school database, each student  1. One-One Relationship (1-1 Relationship) One-to-One (1-1) relationship is defined as the relationship between two tables where both the tables should be associated with each other based on only one matching row. This relationship can be created using Primary key-Unique foreign key constraints. With One-to-One Relationship in SQL Server, for

You're all wrong :) Just kidding.

On a very high load, high volume, heavily updated system splitting a table by 1:1 helps optimize I/O.

For example, this way you can place heavily read columns onto separate physical hard-drives to speed-up parallel reads (the 1-1 tables have to be in different "filegroups" for this). Or you can optimize table-level locks. Etc. Etc.

But this type of optimization usually does not happen until you have millions of rows and huge read/write concurrency

Creating multiple tables and table relationships, Having all our data in one table can make for very difficult data management. We can Selecting rows · Selecting columns and rows · Data vs Schema · Summary · Exercises Thus far in this book, all the work we've done has been with a single database table. What entities might we define for our SQL Book application? There are reasons for and against, and it is rarely black and white until you start modeling the data. Normalized data, generally many-to-many table structures, do the best job of making sure your data elements are correct and stay correct.

Splitting tables into distinct tables with 1:1 relationships between them is usually not practiced, because :

If the relationship is really 1:1, then integrity enforcement boils down to "inserts being done in all concerned tables, or none at all". Achieving this on the server side requires systems that support deferred constraint checking, and AFAIK that's a feature of the rather high-end systems. So in many cases the 1:1 enforcement is pushed over to the application side, and that approach has its own obvious downsides.

A case when splitting tables is nonetheless advisable, is when there are security perspectives, i.e. when not all columns can be updated by one user. But note that by definition, in such cases the relationship between the tables can never be strictly 1:1 .

(I also suggest you read carefully the discussion between Thorsten/Mike. You used the word 'normalization' but normalization has very little to do with your scenario - except if you were considering 6NF, which I think is rather unlikely.)

Learn SQL: Types of relations, This also stands for types of relations between tables. Now, let's write 2 simple SQL SELECT statements and check if this is really true: Compared to previously mentioned types of relations, this one is really rarely used. A One-to-Many relationship is always “owned” by the MANY side, which is to say that the Foreign Key goes in the table for the MANY side (we put the Country identifier in the Person table). Many-to-Many Relationship. A Many-to-Many relationship (use the same two statements to figure this out) is special; it requires a third table. Each of

It makes more sense that your settings are not only in a separate table, but also use a on-to-many relationship between the ID and Settings. This way, you could potentially have a as many (or as few) setting as required.

UserSettings

[Settings_ID]
[User_ID]
[Settings]

In fact, one could make the same argument for the [Email] field.

Types of Relationships :: Chapter 10. Table Relationships :: Part II , A one-to-many relationship exists between a pair of tables when a single record in the first table can be related to one or more records in the second table, but a  A one-to-many relationship exists between two entities if an entity instance in one of the tables can be associated with multiple records (entity instances) in the other table. The opposite relationship does not exist; that is, each entity instance in the second table can only be associated with one entity instance in the first table.

SQL: Practical Guide for Developers, Usually, one-to-one relationships are represented by attributes within a single table. That is, all of the attributes of a table exhibit a natural one-to-one  One-to-Many Relationship. This is an example of a ‘one-to-many’ type of relationship: one value from the “customer_id” column in the “Customers” table can be found many times in the “customer_id” column in the “Sales” table. As a relational schema, this is shown by assigning the correct symbols at the end of the arrow.

How To Model a One-To-One Relationship in a Database (And Why , Want to know what a one-to-one relationship is in a database and how to instead of just Duration: 8:33 Posted: Apr 9, 2019 How can I create a one to one relationship between the tables? I succeeded only in making a One to many relationship: DROP DATABASE if exists test; CREATE DATABASE test; USE test; CREATE TABLE A( a1 int not null, a2 int, PRIMARY KEY (a1) ); CREATE TABLE B( b1 int not null, b2 int, x int not null, PRIMARY KEY (b1,x), FOREIGN KEY (x) references A

Relational Database Design, A relational database organizes data in tables (or relations). A language called SQL (Structured Query Language) was developed to work with relational databases. A primary key is called a simple key if it is a single column; it is called a  Now we have a relationship between the Customers table and the Addresses table. If each address can belong to only one customer, this relationship is "One to One". Keep in mind that this kind of relationship is not very common. Our initial table that included the address along with the customer could have worked fine in most cases.

Comments
  • Does a given Id always have a SettingA, a SettingB and a SettingC? Or can it have just some of them? When an Id has a certain setting, is it always known that it is specifically one of those? Or could it be put under any of them? Were you going to make them nullable? (Your use of "normalize" is dubious (see below). I don't want to make any assumptions from your use of "1:1".)
  • Yes, all users have exactly the same settings.
  • So I guess you mean: yes, no to the or, yes, no to the or, and no. In which respect, then, SettingA, a SettingB and a SettingC are each just like Name and Email.
  • Somewhat related to your question: Why use a 1-to-1 relationship in database design?.
  • You are mostly right, but I would respectfully disagree with your point about making things simpler for the programmer - that's usually done by hiding the "raw" tables behind some sort of API (e.g. views and stored procedures). Having to worry about properly connecting rows from two tables will, if anything, complicate things for the programmer.
  • "You would normally split tables into two or more 1:1 related tables when the table gets very wide . . ." Maybe. But normalization doesn't tell you to do this.
  • OP asked "Is it considered correct to normalize the settings into a separate table . . .".
  • @Mike Sherrill 'Cat Recall': Yes, but you don't normalize anything by splitting the table.
  • That's my point. The OP is clearly confused about what normalization is, and what normalization isn't. Your answer would be more helpful to the OP, and to SO in general, if it corrected his confusion.
  • That is the first step toward the EAV antipattern. And the downside of this first step is you lose the typechecking on the [Settings] column/attribute.
  • Oh, I should have been clearer. I mean that each row be a group of settings, rather than a single attribute. So you could have multiple groups of same-typed settings. Mind you, it still does indeed depend on the nature of the OP's settings in the first place.