1581. Customer Who Visited but Did Not Make Any Transactions

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

Условие

Таблица Visits

Column Name	Type
visit_id	int
customer_id	int
customer_name	varchar

Таблица Transactions

Column Name	Type
transaction_id	int
visit_id	int

Требуется вернуть для каждого customer_id количество его визитов без транзакций (count_no_trans).

Решение

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;