What is a Key Lookup in a Database and what's the difference with Business Rules?

I was wondering what really is a Key lookup in a database ?

And what are the differences between Key Lookup and Busin开发者_高级运维ess Rules?

Thank you guys !

A key lookup is an operator used in a query plan and as a general rule points to a lack of a covering index

http://technet.microsoft.com/en-us/library/bb326635.aspx

I’m not sure about what context you are using business rules in, maybe some kind of check constraint?