Add daily sales report

src/data/courses/sql-mastery/chapters/subqueries-and-ctes/lessons/books-above-average.md

@@ -73,14 +73,3 @@ WHERE price > (
 )
 ORDER BY price DESC;
 ```
-
-This challenge requires you to:
-1. Calculate the average price using a subquery
-2. Compare each book's price to this average
-3. Show how much each book is above the average price
-
-The solution demonstrates the use of subqueries:
-- In the SELECT clause to calculate the difference from average
-- In the WHERE clause to filter for above-average prices
-
----

src/data/courses/sql-mastery/chapters/subqueries-and-ctes/lessons/daily-sales-report.md

@@ -0,0 +1,93 @@
+---
+title: Daily Sales Report
+description: Practice using recursive CTEs to generate dates and analyze daily sales
+order: 145
+type: challenge
+setup: |
+  ```sql
+  CREATE TABLE sale (
+      id INT PRIMARY KEY,
+      sale_date DATE,
+      amount DECIMAL(10,2)
+  );
+
+  INSERT INTO sale (id, sale_date, amount) VALUES
+      (1, '2025-01-01', 99.99),
+      (2, '2025-01-01', 149.99),
+      (3, '2025-01-03', 49.99),
+      (4, '2025-01-05', 199.99),
+      (5, '2025-01-05', 29.99),
+      (6, '2025-01-08', 79.99),
+      (7, '2025-01-10', 159.99),
+      (8, '2025-01-15', 89.99),
+      (9, '2025-01-15', 199.99);
+  ```
+---
+
+The bookstore manager wants a complete daily sales report for the first half of January 2025, including days with no sales. This will help identify patterns in daily sales performance.
+
+Given the following data in table `sale`:
+
+| id | sale_date  | amount |
+| -- | ---------- | ------ |
+| 1  | 2025-01-01 | 99.99  |
+| 2  | 2025-01-01 | 149.99 |
+| 3  | 2025-01-03 | 49.99  |
+| 4  | 2025-01-05 | 199.99 |
+| 5  | 2025-01-05 | 29.99  |
+| 6  | 2025-01-08 | 79.99  |
+| 7  | 2025-01-10 | 159.99 |
+| 8  | 2025-01-15 | 89.99  |
+| 9  | 2025-01-15 | 199.99 |
+
+Write a query using a recursive CTE that shows:
+
+- Date (from January 1st to January 15th, 2025)
+- Number of sales for that date
+- Total amount of sales for that date
+
+Include all dates in the range, even if there were no sales (show 0 for those days).
+
+## Expected Output
+
+| date       | sale_count | total_amount |
+| ---------- | ---------- | ------------ |
+| 2025-01-01 | 2          | 249.98       |
+| 2025-01-02 | 0          | 0.00         |
+| 2025-01-03 | 1          | 49.99        |
+| 2025-01-04 | 0          | 0.00         |
+| 2025-01-05 | 2          | 229.98       |
+| 2025-01-06 | 0          | 0.00         |
+| 2025-01-07 | 0          | 0.00         |
+| 2025-01-08 | 1          | 79.99        |
+| 2025-01-09 | 0          | 0.00         |
+| 2025-01-10 | 1          | 159.99       |
+| 2025-01-11 | 0          | 0.00         |
+| 2025-01-12 | 0          | 0.00         |
+| 2025-01-13 | 0          | 0.00         |
+| 2025-01-14 | 0          | 0.00         |
+| 2025-01-15 | 2          | 289.98       |
+
+## Solution
+
+```sql
+WITH RECURSIVE date_range AS (
+    -- Base case: start with January 1st
+    SELECT DATE '2025-01-01' as date
+    
+    UNION ALL
+    
+    -- Recursive case: add one day until January 15th
+    SELECT date + 1
+    FROM date_range
+    WHERE date < DATE '2025-01-15'
+)
+SELECT 
+    date_range.date,
+    COUNT(s.id) as sale_count,
+    COALESCE(SUM(s.amount), 0.00) as total_amount
+FROM date_range
+LEFT JOIN sale s ON s.sale_date = date_range.date
+GROUP BY date_range.date
+ORDER BY date_range.date;
+```

src/data/courses/sql-mastery/chapters/subqueries-and-ctes/lessons/new-customer-analysis.md

@@ -98,8 +98,6 @@ Write a query using CTEs that shows the top 3 selling books and how many new cus
 | Advanced SQL | 3          | 2                  |
 | Data Science | 3          | 1                  |
 
-> **Note:** Only the top 3 selling books are shown, ordered by total quantity sold.
-
 ## Solution
 
 ```sql