how would you write a sql query to compute a frequency table of a certain attribute involving two joins? What changes would you need to make if you want to order by or group by some attribute? What would you do to account for nulls?
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Answer Posted / Satyendra Anuragi
To compute a frequency table with two joins, use SQL's JOIN and GROUP BY statements. Here is an example using the 'customers', 'orders', and 'order_items' tables where the target attribute is 'product_id'nn```sqlnSELECT order_items.product_id,n SUM(order_items.quantity) as total_quantitynFROM customersnJOIN orders ON customers.customer_id = orders.customer_idnJOIN order_items ON orders.order_id = order_items.order_idnGROUP BY order_items.product_id;n```nTo order by a certain attribute, use the ORDER BY statement at the end of your query like so:n```sqln... ORDER BY total_quantity DESC;n```nTo account for NULL values in your data, you can use the COALESCE function to replace NULLs with zeros or any default value.n```sqln... , COALESCE(order_items.quantity, 0) as quantity ...n```
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