Question

Given a collection of user specified tags how do I determine which ones are not in the tags table with 1 SQL Statement?

Assuming a table schema tags (id, tag) and I’m using mysql, if there’s an optimization I’m unaware of.

thanks

Answer 1

 SELECT  Tag FROM    UserSpecifiedTags   LEFT OUTER JOIN AllTags ON UserSpecifiedTags.Tag = AllTags.Tag WHERE   AllTags.Tag IS NULL 

This should return what you want. In my experience, executing a join and looking for rows which don’t have a match is much quicker than using the IN operator.