LeetCode（数据库）- 每月交易II

题目链接：​​点击打开链接​​

题目大意：略。

解题思路：先将退单表 union all 到交易表，无非是把 state 标记为一个新的状态，比如 cancel，这样两张表可以合并操作，思路就清晰很多。

第二个案例为什么二月份 CB 会没有？

因为此时此地 approved 和 charge 都没有，所以根据题目要求需要过滤掉。

AC 代码

-- 解决方案(1)
WITH t1 AS(SELECT * FROM Transactions

UNION ALL

SELECT trans_id, country, 'cancel' state, amount, c.trans_date
FROM Chargebacks c JOIN Transactions t
ON c.trans_id = t.id),

t2 AS(SELECT DATE_FORMAT(trans_date, '%Y-%m') month, 
country, 
COUNT(IF(state = 'approved', 1, null)) approved_count,
SUM(IF(state = 'approved', amount, 0)) approved_amount,
COUNT(IF(state = 'cancel', 1, null)) chargeback_count,
SUM(IF(state = 'cancel', amount, 0)) chargeback_amount
FROM t1
GROUP BY month, country)

SELECT *
FROM t2
WHERE !(approved_count = 0 AND chargeback_count = 0)

-- 解决方案(2)
SELECT month, country,
COUNT(IF(tag=1, 1, NULL)) AS approved_count,
SUM(IF(tag=1, amount, 0)) AS approved_amount,
COUNT(IF(tag=0, 1, NULL)) AS chargeback_count,
SUM(IF(tag=0, amount, 0)) AS chargeback_amount
FROM (
    SELECT country, amount, 1 AS tag,
    date_format(trans_date, '%Y-%m') AS month
    FROM Transactions
    WHERE state='approved'
    
    UNION ALL

    SELECT country, amount, 0 AS tag,
    date_format(c.trans_date, '%Y-%m') AS month
    FROM Transactions AS t RIGHT OUTER JOIN Chargebacks AS c
    ON t.id = c.trans_id
) AS temp
GROUP BY  month, country;