Everytime a database diagram gets looked out, one area people are critical of is inner joins. They look at them hard and has questions to see if an inner join really needs to be there.

Simple Library Example:

A many-to-many relationship is normally defined in SQL with three tables: Book, Category, BookCategory.

In this situation, Category is a table that contains two columns: ID, CategoryName.

In this situation, I have gotten questions about the Category table, is it need? Can it be used as a lookup table, and in the BookCategory table store the CategoryName instead of the CategoryID to stop from having to do an additional INNER JOIN. (For this question, we are going to ignore the changing, deleting of any CategoryNames)

The question is, what is so bad about inner joins? At what point is doing them a negative thing (general guidelines like # of transactions, # of records, # of joins in a statement, etc)?