Simply think that providing a primary key may satisfy that constraint but they miss that it may violate 3NF

Yes, because a PK is 1NF, not 3NF. There's two more normalizations required before you get to 3NF. :-)

For the moment, ignoring implementation in favor of behavior, if I'm asking for those cities, what's the logical reason for preferring a bag instead of a set? Can you show how this will consistently lead to more correct results?

Let's start with one fact - the SELECT clause is handed a set of tuples and it applies a collection of vertical slices upon said set. So, your question can be rephrased as "What's the reason for not applying set semantics (SELECT DISTINCT) to said slice(s)?"

There are two reasons:

  1. The number of rows in the resultset (which is a set if all the initial tables are sets) shouldn't change depending on what vertical elements you want to see exposed. Think about an Excel spreadsheet. If you hide column D, does the number of rows change? That's all you're doing in a SELECT clause.
  2. The performance hit for applying a DISTINCT is quite high. Try doing a relatively complex query both as SELECT (returning duplicates, as you see them) and SELECT DISTINCT (removing what you see as duplicates) and time them. Now, imagine that on every single query you will ever run in your life. And, no, there aren't that many more performance gains to be made.
My criteria for good software:
  1. Does it work?
  2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?

In reply to Re^5: (OT) Why SQL Sucks (with a little Perl to fix it) by dragonchild
in thread (OT) Why SQL Sucks (with a little Perl to fix it) by Ovid

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