Exam 2010 1B

[Robert Silverflod]

I have  a question regarding the construction of a ER diagram from a schema.

How do a know (from the schema perspective) when there should be many to many or many to one?

As in the question Link -> User, and Entry -> User.

 

[Niklas Broberg]

Your first question for any relation should be, does this relation represent a relationship or an (possibly weak) entity? Here is a non-comprehensive list of things to consider:

1. If a relation has fewer than two references, it cannot represent a relationship, since a relation from a relationship always references both (or all) its related entities.

2. If a relation has a key where not all the attributes included in that key reference other relations, then it cannot represent a relationship. All forms of relationships that translate into relations have as key either the combination of the keys of the related entities (many-to-many), or the key of one of the related entities (the "many" side in a many-to-at-most-one).

3. If a relation has a composite key where its parts reference two (or more) other relations, and all of its parts are included in one such reference, it represents a relationship. An entity must either have a key that has at least some part of it not referencing a different relation, or its key must in entirety reference a single other relation (in the case of a sub-entity). 

4. If a relation has a composite key where part of it references another relation and part of it does not, it represents a weak entity (or in special cases a multi-valued attribute - let's not go there).

5. If a relation representing a relationship has a key that is a composite of all its references, it represents a many-to-many relationship.

6. If a relation representing a relationship has a key that consists of only one of its references, it represents a many-to-at-most-one relationship.

7. If a relation *representing an entity* has an attribute that references another relation, that attribute and reference represent a X-to-one relationship that has been translated away as part of the "X" side. If the attribute in question is part of the key, the relationship was a supporting relationship (see 3). Note that it could represent both a many-to-exactly-one relationship, or a many-to-at-most-one relationship that has been translated using a "nulls" approach.

7b. Note that ISA relationships fall into this category. If the relationship is an ISA, the key of this relation is the same as the reference.

Looking at 2010 1B:

* Users, Links, Entries, Images and Blurbs all represent entities, since they have fewer than two references (see 1).

* Friends represents a relationship (see 3 and 5).

* Comments could be either a sub-entity (see 3) or a many-to-at-most-one relationship (see 6). In fact, those two solutions are equivalent - but conceptually a comment deserves to be an entity.

* Images and Blurbs also represent sub-entities (see 3 and 7b).

* Links represents a weak entity (see 4).

Now the answer to your question follows from 7: The two references you mention both exist on relations that represent entities, then those two references cannot represent many-to-many relationships. 

HTH, Niklas

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