2994. Friday Purchases II 🔒

Description

Table: Purchases

+---------------+------+
| Column Name   | Type |
+---------------+------+
| user_id       | int  |
| purchase_date | date |
| amount_spend  | int  |
+---------------+------+
(user_id, purchase_date, amount_spend) is the primary key (combination of columns with unique values) for this table.
purchase_date will range from November 1, 2023, to November 30, 2023, inclusive of both dates.
Each row contains user id, purchase date, and amount spend.

Write a solution to calculate the total spending by users on each Friday of every week in November 2023. If there are no purchases on a particular Friday of a week, it will be considered as 0.

Return the result table ordered by week of month in ascending order.

The result format is in the following example.

 

Example 1:

Input: 
Purchases table:
+---------+---------------+--------------+
| user_id | purchase_date | amount_spend |
+---------+---------------+--------------+
| 11      | 2023-11-07    | 1126         |
| 15      | 2023-11-30    | 7473         |
| 17      | 2023-11-14    | 2414         |
| 12      | 2023-11-24    | 9692         |
| 8       | 2023-11-03    | 5117         |
| 1       | 2023-11-16    | 5241         |
| 10      | 2023-11-12    | 8266         |
| 13      | 2023-11-24    | 12000        |
+---------+---------------+--------------+
Output: 
+---------------+---------------+--------------+
| week_of_month | purchase_date | total_amount |
+---------------+---------------+--------------+
| 1             | 2023-11-03    | 5117         |
| 2             | 2023-11-10    | 0            |
| 3             | 2023-11-17    | 0            |
| 4             | 2023-11-24    | 21692        |
+---------------+---------------+--------------+ 
Explanation: 
- During the first week of November 2023, transactions amounting to $5,117 occurred on Friday, 2023-11-03.
- For the second week of November 2023, there were no transactions on Friday, 2023-11-10, resulting in a value of 0 in the output table for that day.
- Similarly, during the third week of November 2023, there were no transactions on Friday, 2023-11-17, reflected as 0 in the output table for that specific day.
- In the fourth week of November 2023, two transactions took place on Friday, 2023-11-24, amounting to $12,000 and $9,692 respectively, summing up to a total of $21,692.
Output table is ordered by week_of_month in ascending order.

Solutions

Solution 1: Recursion + Left Join + Date Functions

We can generate a table T that contains all dates in November 2023 using recursion, then use a left join to connect T and the Purchases table by date. Finally, group and sum according to the requirements of the problem.

MySQL

WITH RECURSIVE
    T AS (
        SELECT '2023-11-01' AS purchase_date
        UNION
        SELECT purchase_date + INTERVAL 1 DAY
        FROM T
        WHERE purchase_date < '2023-11-30'
    )
SELECT
    CEIL(DAYOFMONTH(purchase_date) / 7) AS week_of_month,
    purchase_date,
    IFNULL(SUM(amount_spend), 0) AS total_amount
FROM
    T
    LEFT JOIN Purchases USING (purchase_date)
WHERE DAYOFWEEK(purchase_date) = 6
GROUP BY 2
ORDER BY 1;

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