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Unused Accounts

Problem

Table: Accounts

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| account_id    | int     |
| account_name  | varchar |
+---------------+---------+
account_id is the primary key (column with unique values) for this table.
Each row of this table contains the ID and the name of an account in the bank.

Table: Transactions

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| transaction_id| int     |
| account_id    | int     |
| transaction_date | date |
+---------------+---------+
transaction_id is the primary key (column with unique values) for this table.
Each row of this table contains the ID of a transaction, the ID of the account that initiated the transaction, and the date when the transaction was made.

Problem Definition

Write a solution to find all accounts that did not make any transactions in 2020.

Return the result table ordered by account_name in ascending order.

Example

Input: 
Accounts table:
+------------+--------------+
| account_id | account_name |
+------------+--------------+
| 1          | Alice        |
| 2          | Bob          |
| 3          | Charlie      |
+------------+--------------+

Transactions table:
+----------------+------------+-----------------+
| transaction_id | account_id | transaction_date|
+----------------+------------+-----------------+
| 1              | 1          | 2020-09-01      |
| 2              | 2          | 2020-09-02      |
| 3              | 1          | 2020-09-03      |
| 4              | 3          | 2019-08-21      |
| 5              | 2          | 2021-07-03      |
+----------------+------------+-----------------+

Output: 
+--------------+
| account_name |
+--------------+
| Charlie      |
+--------------+

Try It Yourself

java
-- TODO: Write your user queries here

Solution

To identify all accounts that did not make any transactions in the year 2020, we can leverage SQL's LEFT JOIN along with conditional filtering. This approach allows us to include all accounts and exclude those that have associated transactions in the specified year.

SQL Query

SELECT A.account_name
FROM   Accounts AS A
       LEFT JOIN Transactions AS T
              ON A.account_id = T.account_id
                 AND YEAR(transaction_date) = '2020'
WHERE  T.account_id IS NULL
ORDER  BY account_name ASC;

Step-by-Step Approach

Step 1: Perform a Left Join Between Accounts and Transactions for the Year 2020

Combine the Accounts and Transactions tables to associate each account with its transactions in the year 2020. The LEFT JOIN ensures that all accounts are included, even if they have no transactions in 2020.

SQL Query:

SELECT A.account_name, T.account_id
FROM   Accounts AS A
       LEFT JOIN Transactions AS T
              ON A.account_id = T.account_id
                 AND YEAR(transaction_date) = '2020';

Explanation:

Output After Step 1:

Assuming the example input provided, the intermediate result after the LEFT JOIN would be:

+--------------+------------+
| account_name | account_id |
+--------------+------------+
| Alice        | 1          |
| Bob          | 2          |
| Charlie      | NULL       |
+--------------+------------+

Step 2: Filter Accounts Without Transactions in 2020

Identify accounts that did not make any transactions in 2020 by selecting records where the joined Transactions data is NULL.

SQL Query:

SELECT A.account_name
FROM   Accounts AS A
       LEFT JOIN Transactions AS T
              ON A.account_id = T.account_id
                 AND YEAR(transaction_date) = '2020'
WHERE  T.account_id IS NULL;

Explanation:

Output After Step 2:

Based on the intermediate result, the filtered output would be:

+--------------+
| account_name |
+--------------+
| Charlie      |
+--------------+

Step 3: Order the Results by Account Name in Ascending Order

Sort the final list of accounts alphabetically by account_name to present the data in an organized and readable manner.

SQL Query:

ORDER  BY account_name ASC;

Explanation:

Final Output:

+--------------+
| account_name |
+--------------+
| Charlie      |
+--------------+
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