Question

Situation
Say I have the following:

• a table items
• a table lists to which these items can be linked
• a table users

Condition: a list can belong to all or one specific user

What I’d like to do is create “global” lists that exist for ever user. But there should be a possibility to “overwrite” this global list with a user-specific one. Example for a random user:

• a global list called books
• a user-specific list cds

Now when retrieving the lists for this user I should get both the global books lists and the user-specific cds list. But what if this user has a specific list of books?

• a global list called books
• a user-specific list books
• a user-specific list cds

So now it should retrieve the user-specific list called books instead of the global list, and of course the user-specific cds list.

Possible solutions
The easiest thing would be something like this:

table: lists
- id
- name
- userId

If the userId IS NULL it would mean that it’s a global list, but when it’s NOT NULL it belongs to a specific user. So the first example would be for userId = 1:

id  name    userId
1   books   NULL
2   cds     1

To retrieve the lists for this user:

SELECT * FROM lists WHERE userId = 1 OR userId IS NULL

But then I’m stuck at the next part:

id  name    userId
1   books   NULL
2   cds     1
3   books   1

Here the first list “books” is a global one, but this user also has a user-specific list “books”. It is possible to ignore the global list in this case, and only return the user-specific one?

Another possibility would be to create a lists_users table, linking all the users to all the global lists, and deleting this link whenever a user-specific list is created with the same name as one of these global lists. This doesn’t seem very efficient though, and a lot more work to maintain when adding new users.

Question:
What’s the best and most efficient way to set this up in the database, so that I can easily retrieve the correct lists for a specific user?

Answer 1

For your current design you can get user’s list in the following way:

SELECT * FROM lists 
WHERE userId = 1 
UNION ALL
Select * from lists g 
Where userId IS NULL
AND NOT EXISTS (SELECT 1 from lists l where g.name = l.name and userid = 1)

i.e. select user-specific lists and global not-redefined by user