modified some answers

How-to-code-subqueries.sql

@@ -1,8 +1,8 @@
 -- Exercise 1
 
-SELECT DISTINCT category_name
+SELECT category_name
 FROM categories
-WHERE category_id IN (SELECT category_id
+WHERE category_id IN (SELECT DISTINCT category_id
 						FROM products)
 ORDER BY category_name;
 

How-to-code-summary-queries.sql

@@ -7,8 +7,8 @@ FROM orders;
 -- Exercise 2
 
 SELECT category_name,
-	COUNT(products),
-    MAX(list_price)
+	COUNT(product_id) AS count,
+    MAX(list_price) AS max_price
 FROM categories c JOIN products p
 ON c.category_id = p.category_id
 GROUP BY category_name

How-to-create-databases.sql

@@ -1,5 +1,6 @@
 -- exercise 1
 
+SELECT * FROM addresses;
 
 -- exercise 2
 

How-to-retrieve-data-from-a-single-table.sql

@@ -29,7 +29,7 @@ SELECT product_name,
 	list_price,
     discount_percent,
     ROUND((discount_percent/100) * list_price, 2) AS discount_amount,
-    ROUND(list_price - ((discount_percent/100) * list_price), 2) AS discount_price
+    ROUND(list_price - (discount_percent/100) * list_price, 2) AS discount_price
 FROM products
 ORDER BY discount_price DESC
 LIMIT 5;

How-to-retrieve-data-from-two-or-more-tables.sql

@@ -3,7 +3,8 @@
 SELECT category_name,
 	product_name,
     list_price
-FROM categories g JOIN products p
+FROM categories c JOIN products p
+	ON c.category_id = p.category_id
 ORDER BY category_name, product_name ASC;
 
 -- Exercise 2
@@ -52,8 +53,7 @@ ORDER BY last_name, order_date, product_name;
 SELECT p.product_name,
 	p.list_price
 FROM products p JOIN products p2
-	ON p.list_price = p2.list_price
-WHERE p.product_id != p2.product_id
+	ON p.list_price = p2.list_price AND p.product_id <> p2.product_id
 ORDER BY product_name;
 
 -- Exercise 6