3.1. What is a database?#

The introduction to Category theory provides the same list of fundamental concepts behind category theory.

Exercise 3.3.

In both cases, the count is 5. This is not a coincidence because every non-ID column in the two tables represents a reference (internal or external, of which there are 3 and 2 in this case), and every arrow in the graph represents a reference (internal or external) as well. Notice the 5 labels on the 5 arrows in the graph match the 5 column names in the tables.