On which column would you use GROUP BY?

You would use GROUP BY on the columns that define the logical grouping of your data. These are typically the key attributes that identify the categories you want to aggregate over. For example, if you want to find the total sales per customer, you would GROUP BY customer_id. If you want to count orders per day, you would GROUP BY order_date. The columns you choose should be those that you want to preserve as distinct values in the result set, while the other columns are aggregated using functions such as SUM, AVG, COUNT, MAX, or MIN.