Why Group Data?
When you’re working in the Sales Orders database, finding out the number of orders (COUNT), the total sales (SUM), the average of sales (AVG), the smallest order (MIN), or the largest order (MAX) is useful, indeed. And if you want to calculate any of these values by customer, order date, or product, you can add a filter (WHERE) to fetch the rows for one particular customer or product. But what if you want to see subtotals for all customers, displaying the customer name along with the subtotals? To do that, you need to ask your database system to group the rows.
Likewise, in the Entertainment Agency database, it’s easy to find out the number of contracts, the total contract price, the smallest contract price, or the largest contract ...
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