Problem

Table: Users

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+--------------+---------+
| Column Name | Type |
+--------------+---------+
| account | int |
| name | varchar |
+--------------+---------+
account is the primary key (column with unique values) for this table.
Each row of this table contains the account number of each user in the bank.
There will be no two users having the same name in the table.

Table: Transactions

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+---------------+---------+
| Column Name | Type |
+---------------+---------+
| trans_id | int |
| account | int |
| amount | int |
| transacted_on | date |
+---------------+---------+

trans_id is the primary key (column with unique values) for this table. Each row of this table contains all changes made to all accounts. amount is positive if the user received money and negative if they transferred money. All accounts start with a balance of 0.

Write a solution to report the name and balance of users with a balance higher than 10000. The balance of an account is equal to the sum of the amounts of all transactions involving that account.

Return the result table in any order.

The result format is in the following example.

Examples

Example 1

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Input: 
Users table:
+------------+--------------+
| account    | name         |
+------------+--------------+
| 900001     | Alice        |
| 900002     | Bob          |
| 900003     | Charlie      |
+------------+--------------+
Transactions table:
+------------+------------+------------+---------------+
| trans_id   | account    | amount     | transacted_on |
+------------+------------+------------+---------------+
| 1          | 900001     | 7000       |  2020-08-01   |
| 2          | 900001     | 7000       |  2020-09-01   |
| 3          | 900001     | -3000      |  2020-09-02   |
| 4          | 900002     | 1000       |  2020-09-12   |
| 5          | 900003     | 6000       |  2020-08-07   |
| 6          | 900003     | 6000       |  2020-09-07   |
| 7          | 900003     | -4000      |  2020-09-11   |
+------------+------------+------------+---------------+
Output: 
+------------+------------+
| name       | balance    |
+------------+------------+
| Alice      | 11000      |
+------------+------------+
Explanation: 
Alice's balance is (7000 + 7000 - 3000) = 11000.
Bob's balance is 1000.
Charlie's balance is (6000 + 6000 - 4000) = 8000.

Solution

Method 1 – SQL Aggregation and Join

Intuition

Sum all transactions for each account, then join with the Users table and filter for balances greater than 10000.

Approach

  1. Aggregate the Transactions table by account, summing the amount as balance.
  2. Join the result with the Users table on account.
  3. Filter for balance > 10000.
  4. Select name and balance.

Code

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SELECT u.name, t.balance
FROM Users u
JOIN (
  SELECT account, SUM(amount) AS balance
  FROM Transactions
  GROUP BY account
) t ON u.account = t.account
WHERE t.balance > 10000;

Complexity

  • ⏰ Time complexity: O(N + M) where N = number of users, M = number of transactions.
  • 🧺 Space complexity: O(K) for the aggregation result (K = number of unique accounts).