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GROUP BY v.s. Nested Queries
SELECT       product, Sum(price*quantity) AS TotalSales
FROM          Purchase
WHERE       date > “10/1”
GROUP BY  product
SELECT DISTINCT  x.product, (SELECT Sum(y.price*y.quantity)
                                                      FROM     Purchase y
                                                      WHERE x.product = y.product
                                                                   AND y.date > ‘10/1’)
                                                    AS TotalSales
FROM          Purchase x
WHERE       x.date > “10/1”