I added "name" and "homepage" as ideas for additional information you might want to store for each search engine - I should have said that. Having a central location to store that data allows you to easily update the information without having several places where you need to change it.

The engine_id in the table visits points to the search engine that visited you. That useragent is an int in the search engine table is an error - it should be char(250) or something like that.