Let's say we want to display the names of all unique not-null courses.
For each of these courses names
display names of all students enrolled in the course.
Separate the names of students using a comma.
In this case, we need to group the data by course.
However instead of getting an aggregate data from rows grouped by course, we need to combine all names in the group.
SQL provides GROUP_CONCAT, a combination clause over GROUP BY, that solves the exact same problem.
The GROUP_CONCAT function returns the combined data separated by a , by default.
If we want to use a different separator, the GROUP_CONCAT function can take a second argument, to specify the separator.
Check the query below to return a similar result as above, but using a | as a separator.