[MySQL] 리트코드 1581. Customer Who Visited but Did Not Make Any Transactions

문제

Table: Visits
+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| visit_id    | int     |
| customer_id | int     |
+-------------+---------+

visit_id is the column with unique values for this table.
This table contains information about the customers who visited the mall.

Table: Transactions
+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| transaction_id | int     |
| visit_id       | int     |
| amount         | int     |
+----------------+---------+

transaction_id is column with unique values for this table.
This table contains information about the transactions made during the visit_id.

Write a solution to find the IDs of the users who visited without making any transactions and the number of times they made these types of visits.
Return the result table sorted in any order.

 

https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions/description/

예시

Input: 
Visits
+----------+-------------+
| visit_id | customer_id |
+----------+-------------+
| 1        | 23          |
| 2        | 9           |
| 4        | 30          |
| 5        | 54          |
| 6        | 96          |
| 7        | 54          |
| 8        | 54          |
+----------+-------------+
Transactions
+----------------+----------+--------+
| transaction_id | visit_id | amount |
+----------------+----------+--------+
| 2              | 5        | 310    |
| 3              | 5        | 300    |
| 9              | 5        | 200    |
| 12             | 1        | 910    |
| 13             | 2        | 970    |
+----------------+----------+--------+
Output: 
+-------------+----------------+
| customer_id | count_no_trans |
+-------------+----------------+
| 54          | 2              |
| 30          | 1              |
| 96          | 1              |
+-------------+----------------+

Explanation: 
Customer with id = 23 visited the mall once and made one transaction during the visit with id = 12.
Customer with id = 9 visited the mall once and made one transaction during the visit with id = 13.
Customer with id = 30 visited the mall once and did not make any transactions.
Customer with id = 54 visited the mall three times. During 2 visits they did not make any transactions, and during one visit they made 3 transactions.
Customer with id = 96 visited the mall once and did not make any transactions.
As we can see, users with IDs 30 and 96 visited the mall one time without making any transactions. Also, user 54 visited the mall twice and did not make any transactions.

문제 풀이

SELECT  v.customer_id,
        SUM(CASE WHEN t.transaction_id IS NULL THEN 1 END) AS count_no_trans
FROM    visits AS v
LEFT JOIN transactions AS t
        ON v.visit_id = t.visit_id
GROUP BY customer_id
HAVING  count_no_trans IS NOT NULL;

SELECT  v.customer_id,
        COUNT(v.visit_id) AS count_no_trans
FROM    visits AS v
LEFT JOIN transactions AS t
        ON v.visit_id = t.visit_id
WHERE   t.transaction_id IS NULL
GROUP BY v.customer_id;

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SQL을 독학하시는 분들에게 도움이 되길 바라며,

혹 더 좋은 방법이 있거나 오류가 있다면 편하게 말씀 부탁드립니다.