Add monthly sales analysis

src/data/courses/sql-mastery/chapters/scalar-functions/lessons/monthly-sales-analysis.md

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+---
+title: Monthly Sales Analysis
+description: Practice using CASE expressions to analyze monthly book sales
+order: 221
+type: challenge
+setup: |
+  ```sql
+  CREATE TABLE book_sale (
+      id INT PRIMARY KEY,
+      sale_date DATE,
+      total_amount DECIMAL(10,2)
+  );
+
+  INSERT INTO book_sale (id, sale_date, total_amount) VALUES
+      (1, '2024-01-15', 125.50),
+      (2, '2024-01-28', 85.99),
+      (3, '2024-02-03', 45.50),
+      (4, '2024-02-14', 225.00),
+      (5, '2024-02-28', 180.75),
+      (6, '2024-03-05', 95.25),
+      (7, '2024-03-15', 155.50),
+      (8, '2024-03-30', 320.00);
+  ```
+---
+
+The bookstore owner wants to analyze their sales data by month. They need a report showing monthly totals and sales performance.
+
+Given the following data in table `book_sale`:
+
+| sale_date  | total_amount |
+| ---------- | ------------ |
+| 2024-01-15 | 125.50       |
+| 2024-01-28 | 85.99        |
+| 2024-02-03 | 45.50        |
+| 2024-02-14 | 225.00       |
+| 2024-02-28 | 180.75       |
+| 2024-03-05 | 95.25        |
+| 2024-03-15 | 155.50       |
+| 2024-03-30 | 320.00       |
+
+Write a query that shows:
+
+- Month (formatted as 'Month YYYY')
+- Total sales for the month
+- Performance category:
+  - `Excellent` if monthly total > 500
+  - `Good` if monthly total between 200 and 500
+  - `Needs Improvement` if monthly total < 200
+
+## Expected Output
+
+| month         | total_sales | performance |
+| ------------- | ----------- | ----------- |
+| January 2024  | 211.49      | Good        |
+| February 2024 | 451.25      | Good        |
+| March 2024    | 570.75      | Excellent   |
+
+## Solution
+
+```sql
+SELECT
+    TO_CHAR(sale_date, 'Month YYYY') as month,
+    SUM(total_amount) as total_sales,
+    CASE
+        WHEN SUM(total_amount) > 500 THEN 'Excellent'
+        WHEN SUM(total_amount) >= 200 THEN 'Good'
+        ELSE 'Needs Improvement'
+    END as performance
+FROM book_sale
+GROUP BY TO_CHAR(sale_date, 'Month YYYY'), DATE_TRUNC('month', sale_date)
+ORDER BY DATE_TRUNC('month', sale_date);
+```