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Exercise_4.-6.sql

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+SELECT 
+    email_address, order_id, order_date
+FROM
+    customers c
+        JOIN
+    orders o ON c.customer_id = o.customer_id
+WHERE
+    order_date = (SELECT 
+            MIN(order_date)
+        FROM
+            orders
+        WHERE
+            customer_id = o.customer_id);

Exercise_4.4.1.sql

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+SELECT 
+			email_address,
+			o.order_id,
+			SUM((item_price - discount_amount) * quantity) AS order_total
+		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 email_address , order_id 
+       

Exercise_4.5.sql

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+SELECT 
+    product_name, discount_percent
+FROM
+    products
+WHERE
+    discount_percent NOT IN (SELECT 
+            discount_percent
+        FROM
+            products
+        GROUP BY discount_percent
+        HAVING COUNT(discount_percent) > 1)
+ORDER BY product_name;
+
+/* question 5, alternative solution */
+SELECT 
+    product_name, discount_percent
+FROM
+    products p
+WHERE
+    discount_percent NOT IN (SELECT DISTINCT
+            discount_percent
+        FROM
+            products
+        WHERE
+            product_id != p.product_id)
+ORDER BY product_name;