How to model tags in the database?

Question

I have an existing webapp and want to add a tag feature so that users can tag existing objects. The question is should I add a tag column to each object? or should I normalize it and use a tag table where each object will have a collection of tags? I am leaning towards the latter because it feels cleaner, easier to report on and easier to create a tag cloud. But since I know this has been solved 1000 times I wanted to ask and see if I am missing something?

Answer 1

Do you foresee users needing to associate more than one tag with an object?

If not, add the TAG_ID fk to the OBJECT table. Otherwise, you'd need three tables in total to correctly model a many-to-many relationship:

OBJECT

• OBJECT_ID (pk)

OBJECT_TAG_XREF

• OBJECT_ID (pk, fk to OBJECT)
• TAG_ID (pk, fk to TAG)

TAG

• TAG_ID (pk)

Answer 2

Yes, you should normalize it. The 'tag column' is either going to support only one tag per record, or is going to have hideous search performance.

Answer 3

Definitely normalize. A table for tags, a table for your existing objects, and a table of links between them.