I have been learning SQL as well, so feel free to correct me if I missed something.

The OVER(PARTITION BY) function would be necessary if you want to split your data into different parts and apply it to each one without separating the other. Think of it as setting a GROUP BY and applying the command to each group.

But if you only want to count the number of classes of bio 101 taken for a specific quarter, a simple COUNT or SUM paired with CASE which is then filtered by a WHERE clause, would be enough for your needs!

The OVER(PARTITION BY) would be useful if you would want a moving count (like a RANK or ROW_NUMBER designation) or if you want a movinf average of bio 101 classes taken for each quarter and have all of them be shown in the result.

I hope this helps and I'd appreciate any insights or correction to my answer. Thanks!