+------------------+---------+
| Column Name      | Type    | 
+------------------+---------+
| transaction_id   | int     |
| customer_id      | int     |
| transaction_date | date    |
| amount           | decimal |
| transaction_type | varchar |
+------------------+---------+
transaction_id is the unique identifier for this table.
transaction_type can be either 'purchase' or 'refund'.

Input:

customer_transactions table:

+----------------+-------------+------------------+--------+------------------+
| transaction_id | customer_id | transaction_date | amount | transaction_type |
+----------------+-------------+------------------+--------+------------------+
| 1              | 101         | 2024-01-05       | 150.00 | purchase         |
| 2              | 101         | 2024-01-15       | 200.00 | purchase         |
| 3              | 101         | 2024-02-10       | 180.00 | purchase         |
| 4              | 101         | 2024-02-20       | 250.00 | purchase         |
| 5              | 102         | 2024-01-10       | 100.00 | purchase         |
| 6              | 102         | 2024-01-12       | 120.00 | purchase         |
| 7              | 102         | 2024-01-15       | 80.00  | refund           |
| 8              | 102         | 2024-01-18       | 90.00  | refund           |
| 9              | 102         | 2024-02-15       | 130.00 | purchase         |
| 10             | 103         | 2024-01-01       | 500.00 | purchase         |
| 11             | 103         | 2024-01-02       | 450.00 | purchase         |
| 12             | 103         | 2024-01-03       | 400.00 | purchase         |
| 13             | 104         | 2024-01-01       | 200.00 | purchase         |
| 14             | 104         | 2024-02-01       | 250.00 | purchase         |
| 15             | 104         | 2024-02-15       | 300.00 | purchase         |
| 16             | 104         | 2024-03-01       | 350.00 | purchase         |
| 17             | 104         | 2024-03-10       | 280.00 | purchase         |
| 18             | 104         | 2024-03-15       | 100.00 | refund           |
+----------------+-------------+------------------+--------+------------------+

Output:

+-------------+
| customer_id |
+-------------+
| 101         |
| 104         |
+-------------+

Explanation:

• Customer 101:
  • Purchase transactions: 4 (IDs: 1, 2, 3, 4) 
  • Refund transactions: 0
  • Refund rate: 0/4 = 0% (less than 20%) 
  • Active period: Jan 5 to Feb 20 = 46 days (at least 30 days) 
  • Qualifies as loyal 
• Customer 102:
  • Purchase transactions: 3 (IDs: 5, 6, 9) 
  • Refund transactions: 2 (IDs: 7, 8)
  • Refund rate: 2/5 = 40% (exceeds 20%) 
  • Not loyal 
• Customer 103:
  • Purchase transactions: 3 (IDs: 10, 11, 12) 
  • Refund transactions: 0
  • Refund rate: 0/3 = 0% (less than 20%) 
  • Active period: Jan 1 to Jan 3 = 2 days (less than 30 days) 
  • Not loyal 
• Customer 104:
  • Purchase transactions: 5 (IDs: 13, 14, 15, 16, 17) 
  • Refund transactions: 1 (ID: 18)
  • Refund rate: 1/6 = 16.67% (less than 20%) 
  • Active period: Jan 1 to Mar 15 = 73 days (at least 30 days) 
  • Qualifies as loyal 

The result table is ordered by customer_id in ascending order.