Formal Database Modeling Technologies at Wikipedia

Question

I don't understand this table from Wikipedia completely:

row1 1      1   one-to-one                               person ←→ weight
row2 0..1   1   optional on one side one-to-one          date of birth ←→ person
row3 0..*   0..* optional on both sides many-to-many     person ←→ book
row4 1      1..* many-to-one                             person ←→ birth place

row2: I think every person has exactly one date of birth. I don't understand this

row4: I think every person has exactly one birth place. I don't understand this.

What am I missing?

Answer 1

The Wikipedia examples are not the best.

1. Relationships are between entities. The person <-> weight is a poor examples of a 1-1 relationship because weight is not an entity but an attribute. A better example might be employee <-> cube (assuming one-person cubes). Each employee is assigned one and only one cube and each cube be is assigned to one and only one employee.

   In practice, there are few places where you will find 1-1 relationships between tabled entities. You might find them for some PII data that is moved to a separate table for security reasons.

2. Again a poor example and for the same reason: a date is an attribute, not an entity. Use again the employee <-> Cube number example. The optional quality is imparted by making the Cube number field nullable, meaning the cube may not apply or may not be known, at least at the time the record was created. However, it makes more sense to have the optional component on the cube side as a cube could well be empty, not assigned to any employee.

3. A better example. A person may be the author of 0, 1 or many books and a book may be authored by 0 (such as a phone book), 1 or many authors. The optional quality on either side could also mean the data is just not known.

4. Also a fairly good example. A person can only be born in one and only one place -- say, a city. A city, however, can be the birthplace of many people.