many to many and one to many diagrams

Author: louisrli

module2-database-technologies/assets/many-to-many.png

@@ -7,11 +7,21 @@ since one customer has many orders. It should be noted that the foreign key goes
 on the side that is "many". In the example above, note that the `customer_id`
 key is located on the `Order` table.
 
+![One to many relationship](../assets/one-to-many.png)
+
 For a many-to-many relationship, consider students and courses at a university.
 A student can take many courses; a course also has many students. Many-to-many
 relationships are [expressed slightly
 differently](https://dzone.com/articles/how-to-handle-a-many-to-many-relationship-in-datab) than using a single foreign key.
 
+To express a many-to-many relationship between two tables, we create a third
+table that contains the foreign keys of the two tables being connected. For
+example, the diagram below connects classes and students together through a
+table called `Enrollments`. Each row in `Enrollments` represents one class that
+one student is taking; there can be multiple such entries in the table.
+
+![Many to many relationship](../assets/many-to-many.png)
+
 See this [StackOverflow
 question](https://stackoverflow.com/questions/4601703/difference-between-one-to-many-and-many-to-one-relationship)
 for more details.