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Exercise_3.6.sql

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+SELECT 
+    p.product_name,
+    SUM((oi.item_price - oi.discount_amount) * oi.quantity) AS total_amount
+FROM
+    products p
+        JOIN
+    order_items oi ON p.product_id = oi.product_id
+GROUP BY p.product_name ASC WITH ROLLUP;

Exercise_3.7.sql

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+SELECT 
+    c.email_address, 
+    COUNT(DISTINCT oi.product_id) AS number_of_products
+FROM
+    customers c
+        JOIN
+    orders o ON c.customer_id = o.customer_id
+        JOIN
+    order_items oi ON o.order_id = oi.order_id
+GROUP BY c.email_address
+HAVING number_of_products > 1
+ORDER BY c.email_address asc;

Exercise_4.1.sql

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+SELECT DISTINCT
+    category_name
+FROM
+    categories c
+WHERE
+    c.category_id IN (SELECT 
+            category_id
+        FROM
+            products)
+ORDER BY category_name;

Exercise_4.2.sql

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+SELECT 
+    product_name, list_price
+FROM
+    products
+WHERE
+    list_price > (SELECT 
+            AVG(list_price)
+        FROM
+            products)
+ORDER BY list_price DESC;

Exercise_4.3.sql

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+SELECT 
+    category_name
+FROM
+    categories c
+WHERE
+    NOT EXISTS( SELECT 
+            *
+        FROM
+            products p
+        WHERE
+            p.category_id = c.category_id);