fix(sql): joins - resolved the comments

pmikhailov/postgresql/joins.sql

@@ -44,7 +44,7 @@ FROM
 LEFT JOIN 
 	order_items oi ON o.id =  oi.order_id 
 INNER JOIN 
-	users u ON o.id = u.id
+	users u ON o.user_id = u.id
 INNER JOIN
 	products p ON oi.product_id = p.id;
 
@@ -69,7 +69,7 @@ SELECT
 FROM
 	users u 
 LEFT JOIN
-	orders o ON u.id = o.id
+	orders o ON u.id = o.user_id
 WHERE o.user_id IS NOT NULL