Saturday, July 2, 2016

This Week

1. What's wrong with this picture?

AT: Well, I think I am a bit confused now. In my personal understanding, a relation is defined as a set of tuples. Then ... "in the relational model every relation represents a relationship". And then a quote from Chen: "each tuple of entities ... is a relationship". If I use the first and the second statements - I can say that a relationship is a set of tuples. The third statement says that a relationship is a tuple. So far, is a relationship a set of an element of a set? (Or may be a set of sets?)
GE: I argue that there is essentially no difference between relationships between entity (type tables) and between an entity and its attributes. They both represent relationships between two populations of things. Something is an attribute by virtue of there being a relationship. If relationships are represented by foreign keys and the entity tables must be in 1NF, as in the relational model, then all relationships must be at most Many-to-One (a very unnecessary limitation when modeling some user domain).
TF: The relational model was a mathematical construct, derived from set theory. Hence that particular terminology. The entity-relationship model is essentially a directed graph model, where relationships are prominent residents. Not so in the relational model (despite the name), where relationships (between relations, mind you) are not visible and in the SQL implementations is reduced to constraints. Relationships are about structure, which is as important as meaning (the semantics of the terms used in the universe being modeled).
2. Quote of the Week
"In Relational Theory sometimes the relationships, where we do our Joins are much more important than the attributes on an Entity." (quoted in exchange)

3. To Laugh or Cry? 
4. Of Interest

And now for something completely different

The PostWest (The future of the West is all behind it)

Pinch me of the week
Consider this 
in light of this:

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  1. AT fails to appreciate the distinction between relations per RM and relationships per ER. With all the usual consequences.

    GE writes, as usual, from a mindset in which conceptual modeling bleeds/leaks over into the logical level modeling. E.g. "if relationships [conceptual] are represented by foreign keys [logical]".

    TF obviously has the heart in the right place, but goofs in his last sentence. What he says there is true of relations per RM as well. Or perhaps even arguably, those things ("... are about structure ...") are true only of relations, not relationships.

    1. > AT fails to appreciate the distinction between relations per RM and relationships per ER.

      What I call conceptual-logical conflation (CLC). In fact, in an older LI exchange GE said that a weakness of the RDM is that you cannot enforce a M:M relationship without dual referential constraints, as I wrote in a recent post.