sql - To prevent the use of duplicate Tags in a database

Question

I would like to know how you can prevent to use of two same tags in a database table. One said me that use two private keys in a table. However, W3Schools -website says that it is impossible.

My relational table

alt text http://files.getdropbox.com/u/175564/db/db7.png

My logical table

alt text http://files.getdropbox.com/u/175564/db/db77.png

The context of tables

alt text http://files.getdropbox.com/u/175564/db/db777.png

How can you prevent the use of duplicate tags in a question?

Answer 1

You can create a unique constraint on (question_id, tag_name) in the tags table, which will ensure that the pair is unique. That would mean that the same question may not have the same tag attached more than once. However, the same tag could still apply to different questions.