There's not enough data in northwind as it is to properly test this and you must test this like the author suggested using multiple ( optional ) parameters - testing with one where clause does not fully illustrate the issues, the idea of using this approach, quite correctly and I agree with Alex's solution, is that you wish to typically deal with many parameters , please choose at least 6 for testing, which may or may not be passed thus making multiple procs ( one to match each combination ) or multiple if statements unworkable. Then see how the performance degrades with respect to data size and how difficult it is to index. And don't always just have one matching result per where clause - real life often isn't like that.

btw - good one Alex you've sparked an interesting thread!