Database normalization and duplication

Question

I have a many to many relationship between bookmarks and tags:

• Bookmarks
  • Id
  • UserId
  • Title
  • Url
• Tags
  • Id
  • UserId
  • Title
  • Description
• TagsBookmarks (junction table)
  • TagId
  • BookmarkId

As far as I know this is normalized and valid.

However it is possible that multiple users save the same bookmarks which means duplicate URLs in the bookmarks table. Does this kind of duplication belong to normalization and should it be avoided?

Update:

The system works like this:

• User logs in and create bookmarks / tags.

• A bookmark should have atleast one tag attached to it. For example: "http://google.com" may belong to tag "google" and "search-engine".

• When a bookmark gets deleted the relation in TagsBookmarks also get deleted. The tags do not get deleted.

• A user can delete a tag if it has no bookmarks.

• A bookmark cannot exist without a tag.

• A tag can exist without bookmarks.

Answer 1

Usually, table names and table columns are singular.

I would move UserID to TagBookmark.

Bookmark
--------
BookmarkID
BookmarkTitle
BookmarkURL

Tag
---
TagID
TagTitle
TagDescription

TagBookmark
-----------
UserID
TagID
BookMarkID

The primary (clustering) key of TagBookmark would be all 3 ID fields concatenated.

I'm not sure what the difference is between a TagTitle and a BookmarkTitle, so there may be some further normalization to be done.

Answer 2

You should exclude UserId from Bookmarks and Tags tables and add it to your junction table, because your system is user-centric as I understood from your explanation. Like this:

• Users
  • Id
  • FirstName
  • ...
• Bookmarks
  • Id
  • Title
  • Url
• Tags
  • Id
  • Title
  • Description
• UsersTagsBookmarks (junction table)
  • UserId
  • TagId
  • BookmarkId

or even you need two junction tables, but it depends on your system logics:

• TagsBookmarks (junction table 1, specifies all possible tags/bookmarks combinations)
  • Id
  • TagId
  • BookmarkId
• UsersTagsBookmarks (junction table 2)
  • UserId
  • TagBookmarkId