doc: How to code summary querries

How_to_code_summary_querries.sql

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+/*1*/
+SELECT
+    count(*) AS order_count,
+    SUM(tax_amount) AS tax_total
+FROM
+	orders;
+
+/*2*/
+SELECT
+	category_name,
+	COUNT(product_id) AS product_count,
+    MAX(list_price) AS most_expensive_product
+FROM 
+	products p
+JOIN
+	categories c
+ON
+	p.category_id = c.category_id
+GROUP BY 
+	category_name
+ORDER BY COUNT(product_id) DESC, category_name DESC;
+
+/*3*/
+SELECT
+	email_address,
+    SUM(item_price) * quantity AS item_price_total ,
+    SUM(discount_amount) * quantity AS discount_amount_total
+FROM 
+	orders o  
+JOIN 
+	order_items oi
+ON 
+	o.order_id = oi.order_id
+JOIN 
+	customers c
+ON 
+	o.customer_id = c.customer_id
+GROUP BY 
+	c.customer_id
+ORDER BY 
+	SUM(item_price) * quantity DESC;
+
+/*4*/
+SELECT
+	c.email_address,
+	COUNT(oi.order_id) AS 'order_count',
+	SUM(item_price - discount_amount) * quantity AS 'order_total'
+FROM orders ord JOIN order_items oi
+ON o.order_id = oi.order_id
+JOIN customers c
+ON o.customer_id = c.customer_id
+GROUP BY email_address
+HAVING COUNT(oi.order_id) > 1
+ORDER BY SUM(item_price - discount_amount) DESC, COUNT(oi.order_id);
+
+/*5*/
+SELECT
+	email_address,
+    COUNT(o.order_id) AS 'order_count',
+    SUM(item_price - discount_amount) * quantity AS 'order_total'
+FROM orders o  
+JOIN order_items oi
+	ON o.order_id = oi.order_id
+    JOIN customers c
+    ON o.customer_id = c.customer_id
+WHERE oi.item_price > 400
+GROUP BY email_address
+HAVING COUNT(oi.order_id) > 1
+ORDER BY SUM(item_price - discount_amount) * COUNT(oi.order_id)  DESC;
+
+
+/*6*/ /* kan het dat 1 bedrag niet klopt*/
+SELECT
+	product_name,
+    SUM(item_price - discount_amount) * quantity AS 'Total Amount'
+FROM 
+	products p 
+JOIN 
+	order_items o 
+ON 
+	p.product_id = o.product_id
+GROUP BY 
+product_name WITH ROLLUP;
+
+/*7*/
+SELECT 
+	email_address,
+    COUNT(product_id) AS 'number_of_products'
+FROM 
+	customers c 
+JOIN 
+	orders o
+ON 
+	c.customer_id = o.customer_id
+JOIN 
+	order_items ord 
+ON 
+	o.order_id = ord.order_id
+GROUP BY 
+	email_address
+HAVING 
+	COUNT(o.order_id) > 1;