Multiple users but storing data in same tables

Question

Hi I am new to databases in general, so forgive my noob question below, but I really do need help.

I have designed a database with 4 tables. I have created an application in C# which will store some values in these 4 tables during the course of the application running. However, this is all working fine when there is only one user of the application, but if more that one user is going to use the same application running in an ASP.NET page, then they will be accessing and altering the data in the 4 tables, and problems will soon arise.

My question is, how do I prevent this from happening? I want each user to have their own unique username and then use that to differentiate them in the tables but my knowledge of databases is limited to know how to achieve this. Please help?

Answer 1

Supposing you have the following table today:

FavoriteFood
—————————————
FoodId
FoodName

And it lists all of your favorite foods. But then you decide you'll let me use your database to store my favorite foods too. Since you don't care about my favorite foods, and I don't care about yours, you need a way of keeping them separate. First, you'll create a User table:

User
—————————
UserId
UserName
FirstName
LastName

Then, you need to relate the User table to the FavoriteFood table. One way of doing this would be to add a Foreign Key to the FavoriteFood table. Give it a new field called UserId :

FavoriteFood
—————————————
FoodId
UserId
FoodName

Then you can get just the food for a single user by adding a WHERE clause to your SQL code:

SELECT FoodName
  FROM FavoriteFood
 WHERE UserId = @UserId

That could be ok, but I'm not satisfied with it. This database is not normalized! Suppose, you later want to store calorie information about your foods. You add a field called calories to your FavoriteFoods table. As you are populating that field with data, you notice that you are putting in the same data multiple times. All of your users like bananas, so you have as many entries in your table for bananas as you have users. You have to enter the exact same calorie information over and over again. Instead, you should have all the information for a food in the table just once, and use a completely separate table to map food to users:

Food
—————————
FoodId
FoodName
Calories

FavoriteFood
—————————————
FoodId
UserId

Use a join to get the favorite food for a user:

SELECT f.FoodName
      ,f.Caloires
  FROM Food f
  JOIN FavoriteFood a ON a.FoodId = f.FoodId
 WHERE a.UserId = @UserId