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Formatted question description: https://leetcode.ca/all/1336.html

1336. Number of Transactions per Visit

Level

Hard

Description

Table: Visits

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| visit_date    | date    |
+---------------+---------+
(user_id, visit_date) is the primary key for this table.
Each row of this table indicates that user_id has visited the bank in visit_date.

Table: Transactions

+------------------+---------+
| Column Name      | Type    |
+------------------+---------+
| user_id          | int     |
| transaction_date | date    |
| amount           | int     |
+------------------+---------+
There is no primary key for this table, it may contain duplicates.
Each row of this table indicates that user_id has done a transaction of amount in transaction_date.
It is guaranteed that the user has visited the bank in the transaction_date.(i.e The Visits table contains (user_id, transaction_date) in one row)

A bank wants to draw a chart of the number of transactions bank visitors did in one visit to the bank and the corresponding number of visitors who have done this number of transaction in one visit.

Write an SQL query to find how many users visited the bank and didn’t do any transactions, how many visited the bank and did one transaction and so on.

The result table will contain two columns:

  • transactions_count which is the number of transactions done in one visit.
  • visits_count which is the corresponding number of users who did transactions_count in one visit to the bank.

transactions_count should take all values from 0 to max(transactions_count) done by one or more users.

Order the result table by transactions_count.

The query result format is in the following example:

Visits table:
+---------+------------+
| user_id | visit_date |
+---------+------------+
| 1       | 2020-01-01 |
| 2       | 2020-01-02 |
| 12      | 2020-01-01 |
| 19      | 2020-01-03 |
| 1       | 2020-01-02 |
| 2       | 2020-01-03 |
| 1       | 2020-01-04 |
| 7       | 2020-01-11 |
| 9       | 2020-01-25 |
| 8       | 2020-01-28 |
+---------+------------+
Transactions table:
+---------+------------------+--------+
| user_id | transaction_date | amount |
+---------+------------------+--------+
| 1       | 2020-01-02       | 120    |
| 2       | 2020-01-03       | 22     |
| 7       | 2020-01-11       | 232    |
| 1       | 2020-01-04       | 7      |
| 9       | 2020-01-25       | 33     |
| 9       | 2020-01-25       | 66     |
| 8       | 2020-01-28       | 1      |
| 9       | 2020-01-25       | 99     |
+---------+------------------+--------+
Result table:
+--------------------+--------------+
| transactions_count | visits_count |
+--------------------+--------------+
| 0                  | 4            |
| 1                  | 5            |
| 2                  | 0            |
| 3                  | 1            |
+--------------------+--------------+
* For transactions_count = 0, The visits (1, "2020-01-01"), (2, "2020-01-02"), (12, "2020-01-01") and (19, "2020-01-03") did no transactions so visits_count = 4.
* For transactions_count = 1, The visits (2, "2020-01-03"), (7, "2020-01-11"), (8, "2020-01-28"), (1, "2020-01-02") and (1, "2020-01-04") did one transaction so visits_count = 5.
* For transactions_count = 2, No customers visited the bank and did two transactions so visits_count = 0.
* For transactions_count = 3, The visit (9, "2020-01-25") did three transactions so visits_count = 1.
* For transactions_count >= 4, No customers visited the bank and did more than three transactions so we will stop at transactions_count = 3

The chart drawn for this example is as follows:

Image text

Solution

For each entry in the result table, the value of transactions_count may be greater than 0 or equal to 0.

For transactions_count greater than 0, find the maximum number of transactions, use a variable to generate each transaction count from 1 to the maximum number of transactions.

For transactions_count equal to 0, deal with the case individually.

Finally, use union all to union the two parts of results.

# Write your MySQL query statement below
select t5.transactions_count, ifnull(t4.count_sum, 0) as visits_count from (
    select @num := @num + 1 as transactions_count from Transactions, (select @num := 0) t2
        where @num < (
            select max(t1.count) as max_visit from (
                select user_id, transaction_date, count(*) as count from Transactions
                    group by user_id, transaction_date
            ) t1
        )
) t5
left join (
    select t3.count, count(*) as count_sum from (
        select user_id, transaction_date, count(*) as count from Transactions
            group by user_id, transaction_date
    ) t3 group by t3.count
) t4 on t4.count = t5.transactions_count
union all
select 0 as transactions_count, count(*) as visits_count from (
    select user_id, transaction_date, sum(amount) as amount from Transactions
        group by user_id, transaction_date
) t6
    right join Visits v
    on t6.user_id = v.user_id
    and t6.transaction_date = v.visit_date
    where t6.amount is null
    order by transactions_count asc;

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