Find all the courses in which at least one student is enrolled.
The problem seems easy enough.
We need to list items from the course columns.
In the result we can see Python 3 times.
Similarly Java is also appearing more than once.
That's not what we wanted.
We want a unique list of all courses which have at least one student enrolled in.
This is what DISNTICT does.
DISTINCT always returns a unique list.
Result returned by DISTINCT won't have an item more than once.