Problem

Table: Transactions

+—————+———+ | Column Name | Type | +—————+———+ | id | int | | country | varchar | | state | enum | | amount | int | | trans_date | date | +—————+———+ id is the primary key of this table. The table has information about incoming transactions. The state column is an enum of type [“approved”, “declined”].

Write an SQL query to find for each month and country, the number of transactions and their total amount, the number of approved transactions and their total amount.

Return the result table in any order.

The query result format is in the following example.

Examples

Example 1

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Input: 
Transactions table:
+------+---------+----------+--------+------------+
| id   | country | state    | amount | trans_date |
+------+---------+----------+--------+------------+
| 121  | US      | approved | 1000   | 2018-12-18 |
| 122  | US      | declined | 2000   | 2018-12-19 |
| 123  | US      | approved | 2000   | 2019-01-01 |
| 124  | DE      | approved | 2000   | 2019-01-07 |
+------+---------+----------+--------+------------+
Output: 
+----------+---------+-------------+----------------+--------------------+-----------------------+
| month    | country | trans_count | approved_count | trans_total_amount | approved_total_amount |
+----------+---------+-------------+----------------+--------------------+-----------------------+
| 2018-12  | US      | 2           | 1              | 3000               | 1000                  |
| 2019-01  | US      | 1           | 1              | 2000               | 2000                  |
| 2019-01  | DE      | 1           | 1              | 2000               | 2000                  |
+----------+---------+-------------+----------------+--------------------+-----------------------+

## Solution

### Method 1 - SQL Aggregation

#### Intuition
We need to aggregate transaction counts and amounts by month and country, and also count and sum approved transactions separately. Use conditional aggregation for approved transactions.

#### Approach
Group by month and country, count all transactions, sum all amounts, and use CASE WHEN for approved transactions.

#### Code



    

         
         
         
        
            
            
        
            
            
                 
                 
                
            
        
            
            
        
    


    

        
         
        
        
         
         
        

             
            
             
            
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
                 
                     
                
            
            
            
            




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SELECT DATE_FORMAT(trans_date, '%Y-%m') AS month,
       country,
       COUNT(*) AS trans_count,
       SUM(state = 'approved') AS approved_count,
       SUM(amount) AS trans_total_amount,
       SUM(CASE WHEN state = 'approved' THEN amount ELSE 0 END) AS approved_total_amount
FROM Transactions
GROUP BY month, country





        

        
        
         
        
        
    





#### Complexity

* ⏰ Time complexity: `O(N)` — Each row is processed once per aggregation.
* 🧺 Space complexity: `O(M)` — For result groups, M = number of unique (month, country) pairs.