Using group by on multiple columns

Question

I recognize the purpose of GROUP BY x.

However, how do GROUP BY X, Y operate and what does it signify?

Answer 1

Group By X means to put all those with the same value for X in one group.

Group By X, Y means to put all those with the same values for both X and Y in one group.

Let's use the following table, which pertains to which university students are enrolled in which subjects, as an illustration:

Table: Subject_Selection

+---------+----------+----------+
| Subject | Semester | Attendee |
+---------+----------+----------+
| ITB001  |        1 | John     |
| ITB001  |        1 | Bob      |
| ITB001  |        1 | Mickey   |
| ITB001  |        2 | Jenny    |
| ITB001  |        2 | James    |
| MKB114  |        1 | John     |
| MKB114  |        1 | Erica    |
+---------+----------+----------+

When you use a group by on the subject column only; say:

select Subject, Count(*)
from Subject_Selection
group by Subject

Output:

+---------+-------+
| Subject | Count |
+---------+-------+
| ITB001  |     5 |
| MKB114  |     2 |
+---------+-------+

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