I am trying to follow the suggestions made in this post I want to set a combination of (skiposttag_skipost_id and skiposttab_tag_id) as primary key. In this case it will guarantee that a tag can be mentioned only once for the given article.

I thought this design would do it but as you can see the same article contains the same tag id. How can I enforce that for each skiposttag_skipost_id there is also only a unique tag (used once only) in skiposttag_tag_id please?