Scaling Web Applications and Tagging - Digg, Del.icio.us, StackOverflow

Question

How do websites like Digg, Del.icio.us, and StackOverflow implement tagging?

I know this other question has an accepted answer of a many-to-many relation with a cros

Answer 1

I am sure that the additional JOIN queries would be too expensive in a very large system.

The tags are either stored non-normalised in the main table, or there may be a separate tag table which has a row for each tagged item.