LC 1581进店却未进行过交易的顾客Customer Who Visited but Did Not Make Any Transactions　简单

Visits 记录顾客到店，Transactions 记录到店产生的交易；统计每位只到店、无交易的顾客的次数。

思路 LEFT JOIN 后交易为 NULL 即无交易，GROUP BY 计数。

SELECT V.customer_id, COUNT(*) AS count_no_trans
FROM Visits V
LEFT JOIN Transactions T ON V.visit_id = T.visit_id
WHERE T.transaction_id IS NULL
GROUP BY V.customer_id;

数据表：Visits,Transactions　·　关键字：统计每位只到店、无交易的顾客的次数