solution

solution.sql

@@ -0,0 +1,31 @@
+-- Section1
+CREATE INDEX idx_orders_created_at ON orders (created_at,total);
+SELECT SUM(total)
+FROM orders
+WHERE created_at BETWEEN '2020-01-01 00:00:00' AND '2020-12-31 23:59:59';
+
+-- Section2
+create index idx_orders_user on orders (user_id,created_at,total);
+SELECT SUM(total)
+FROM orders
+WHERE created_at BETWEEN '2020-01-01 00:00:00' AND '2020-12-31 23:59:59'
+AND user_id = 345;
+
+-- Section3
+WITH RECURSIVE cte AS (
+    SELECT MIN(DATE(created_at)) AS date
+    FROM orders
+    UNION ALL
+    SELECT DATE_ADD(cte.date, INTERVAL 1 DAY)
+    FROM cte
+    WHERE DATE_ADD(cte.date, INTERVAL 1 DAY) <= (SELECT MAX(created_at) FROM orders)
+)
+SELECT DATE(cte.date) AS date,
+       SUM(IFNULL(orders.total, 0)) AS total_value
+FROM cte
+LEFT JOIN (
+    SELECT DATE(created_at) AS order_date, total AS total
+    FROM orders
+) AS orders ON cte.date = orders.order_date
+GROUP BY cte.date
+ORDER BY cte.date ASC;