Question

I have a database with two tables: Users and Categories.

Users has these fields:

UserId        unique identifier
UserName      nvarchar
CategoryId    int

Categories has these fields:

CategoryId    int
CategoryName  nvarchar

At the moment, every user is in one category. I want to change this so that each user can be in any number of categories. What is the best way to do this?

My site does a lot of really expensive searches, so the solution needs to be as efficient as possible. I don’t really want to put the list of categories each user has in a third table, as this means that when I pull back a search, each user will be represented in several rows at once (at least, this is what would happen if I implemented a search with my current, fairly crude, understanding of sql.)

EDIT:

If setting up a many-many relationship, is it possible to return only one row for each user?

For instance:

DECLARE @SearchUserID nvarchar(200) = 1;

SELECT *
FROM Users JOIN Categories JOIN CategoriesPerUser
WHERE UserId = @SearchUserID

This would return one row for each category the user belonged to. It is possible to have it only return one row?

Answer 1

At the moment you have a one-to-many relationship, that is to say category can be assocaited with many users, but a user can only be assocaited with one category.

You need to change this to a many-to-many relationship so that each user can be assocaited with many categories and each category can be assocaited with many users.

This is achieves by adding a table which links a userid and a category id (and removing categoryid from the user table)

Table: UserToCategory
UserId     int
CategoryId int

As for your last paragraph, this is the most efficient way of modelling your requirement. You should make a combination of UserId/CategoryId the PrimaryKey in this table to stop a user being associated with the same category twice. This stops the problem of a user returned twice for a particular category.

The SQL to find, for example, all users associated with a category would be

SELECT u.*
FROM Users u
INNER JOIN UserToCategory uc 
   ON u.UserId = uc.UserID
WHERE uc.CategoryId = 123

---

Edit after comments: If you have a query that finds a number of users, and you want a distinct list of categories associated with those users this could be done like

SELECT c.*
FROM Categories c
WHERE CategoryId IN
(
  SELECT uc.CategoryID
  FROM UserToCategory uc
    INNER JOIN Users u ON uc.UserId = u.UserID
  WHERE <some criteria here to filter users>
)