How can excess rows be removed for each group?

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To remove excess rows for each group, you can use a combination of the GROUP BY clause and the HAVING clause in SQL.

The GROUP BY clause groups the data by a specific column or columns, and the HAVING clause filters the grouped data based on a condition.

For example, the following SQL query groups a table of customer orders by customer ID and product ID and selects only the rows where the customer has ordered the maximum number of products for each product:

SELECT customer_id, product_id, MAX(quantity) as max_quantity
FROM orders
GROUP BY customer_id, product_id
HAVING quantity = MAX(quantity)

This will return one row for each customer and product combination, showing the customer ID, product ID, and the maximum quantity they ordered for that product. Any rows where the quantity is not the maximum for that customer and product combination will be excluded from the result.