update: question faizanxmulla#12 solution

Author: faizanxmulla

ankit-bansal-sql/complex-sql-questions-playlist/12-total-sales-amount-by-year.md

@@ -21,42 +21,49 @@ INSERT INTO product (product_id, product_name) VALUES
 
 CREATE TABLE sales (
     product_id INT,
-    period_start VARCHAR(20),
+    period_start DATE,
     period_end DATE,
-    average_daily_sales INT
+    average_daily_sales INT,
+    FOREIGN KEY (product_id) REFERENCES Product(product_id)
 );
 
-INSERT INTO sales (product_id, period_start, period_end, average_daily_sales) VALUES
-(1, '2023-07-01', '2023-07-31', 150),
-(2, '2023-06-01', '2023-06-30', 45),
-(3, '2023-05-01', '2023-05-31', 2),
-(1, '2023-01-01', '2023-01-31', 140),
-(2, '2022-12-01', '2022-12-31', 60),
-(3, '2023-02-01', '2023-02-28', 5);
+INSERT INTO Sales (product_id, period_start, period_end, average_daily_sales) VALUES
+(1, '2019-01-25', '2019-02-28', 100),
+(2, '2018-12-01', '2020-01-01', 10),
+(3, '2019-12-01', '2020-01-31', 1);
 ```
 
+### Expected Output
+
+
+| product_id | report_year | total_amount |
+|------------|-------------|--------------|
+| 1          |    2019     | 3500         |
+| 2          |    2018     | 310          |
+| 2          |    2019     | 3650         |
+| 2          |    2020     | 10           |
+| 3          |    2019     | 31           |
+| 3          |    2020     | 31           |
+
+
 
 ### Solution Query
 
 ```sql
-SELECT    b.product_id,
-          a.product_name,
-          a.yr AS report_year,
-          CASE 
-              WHEN YEAR(b.period_start) = YEAR(b.period_end) AND a.yr = YEAR(b.period_start) THEN DATEDIFF(b.period_end,b.period_start) + 1
-              WHEN a.yr = YEAR(b.period_start) THEN DATEDIFF(DATE_FORMAT(b.period_start,'%Y-12-31'), b.period_start)+1
-              WHEN a.yr = YEAR(b.period_end) THEN DAYOFYEAR(b.period_end) 
-              WHEN a.yr > YEAR(b.period_start) AND a.yr < YEAR(b.period_end) THEN 365
-              ELSE 0
-          END * average_daily_sales AS total_amount
-FROM (
-      SELECT product_id,product_name,'2018' AS yr FROM Product
-      UNION
-      SELECT product_id,product_name,'2019' AS yr FROM Product
-      UNION
-      SELECT product_id,product_name,'2020' AS yr FROM Product
-	) a
-    JOIN Sales b ON a.product_id=b.product_id  
-HAVING   total_amount > 0
-ORDER BY 1, 3
+-- easy solution using RECURSIVE CTE.
+
+WITH RECURSIVE all_dates AS (
+    SELECT MIN(period_start) as dates, MAX(period_end) as max_date
+    FROM   sales
+    UNION ALL
+    SELECT dates + 1 as dates, max_date
+    FROM   all_dates
+    WHERE  dates < max_date
+)
+SELECT   product_id, 
+         EXTRACT(YEAR FROM dates) as report_year, 
+         SUM(average_daily_sales) as total_amount
+FROM     all_dates ad JOIN sales s ON dates BETWEEN period_start AND period_end 
+GROUP BY 1, 2
+ORDER BY 1, 2
 ```