Challenge: Use Joins to Combine Rows

Problem statement

1. How many students are enrolled in each course?
2. There are five students, but only four are currently registered in any courses. Write a query to show this information.

The students sample table is below:

+----+--------+---------------------+----------------------+
| id | name   | address             | date_of_registration |
+----+--------+---------------------+----------------------+
|  1 | John   |

...