You want to normalize the data as much as you need to. There really isn't a "too far" or a "not far enough." Meet the business requirements in the best way possible. Remember that normalization not only increases data accuracy, but it reduces the amount of data stored. For example, you can create all the address information with 50 million customers, repeating addresses over and over again, or you can link to an address table and radically reduce the amount of data stored. That two table join is not going to seriously impact performance. Three, four, and 15 table joins won't seriously impact performance either if you've got good indexes, especially good clustered indexes. Flattening the structure reduces joins and simplifies queries, but it could make for poorer performance (you'll need to index more columns on the table and maintain that data on that one table with more page splits, more index rebuilds...). If flat files were better, we'd never have gone to relational databases in the first place.