Leetcode1205. 每月交易II（中等）

题目

Transactions 记录表

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| id             | int     |
| country        | varchar |
| state          | enum    |
| amount         | int     |
| trans_date     | date    |
+----------------+---------+

id 是这个表的主键。
该表包含有关传入事务的信息。
状态列是类型为 [approved（已批准）、declined（已拒绝）] 的枚举。

Chargebacks 表

+----------------+---------+
| Column Name    | Type    |
+----------------+---------+
| trans_id       | int     |
| charge_date    | date    |
+----------------+---------+

退单包含有关放置在事务表中的某些事务的传入退单的基本信息。
trans_id 是 transactions 表的 id 列的外键。
每项退单都对应于之前进行的交易，即使未经批准。

编写一个 SQL 查询，以查找每个月和每个国家/地区的已批准交易的数量及其总金额、退单的数量及其总金额。

注意：在您的查询中，给定月份和国家，忽略所有为零的行。

查询结果格式如下所示：

Transactions 表：

+------+---------+----------+--------+------------+
| id   | country | state    | amount | trans_date |
+------+---------+----------+--------+------------+
| 101  | US      | approved | 1000   | 2019-05-18 |
| 102  | US      | declined | 2000   | 2019-05-19 |
| 103  | US      | approved | 3000   | 2019-06-10 |
| 104  | US      | approved | 4000   | 2019-06-13 |
| 105  | US      | approved | 5000   | 2019-06-15 |
+------+---------+----------+--------+------------+

Chargebacks 表：

+------------+------------+
| trans_id   | trans_date |
+------------+------------+
| 102        | 2019-05-29 |
| 101        | 2019-06-30 |
| 105        | 2019-09-18 |
+------------+------------+

Result 表：

+----------+---------+----------------+-----------------+-------------------+--------------------+
| month    | country | approved_count | approved_amount | chargeback_count  | chargeback_amount  |
+----------+---------+----------------+-----------------+-------------------+--------------------+
| 2019-05  | US      | 1              | 1000            | 1                 | 2000               |
| 2019-06  | US      | 3              | 12000           | 1                 | 1000               |
| 2019-09  | US      | 0              | 0               | 1                 | 5000               |
+----------+---------+----------------+-----------------+-------------------+--------------------+

解答
先两表连接

SELECT *
FROM Transactions AS T
LEFT JOIN Chargebacks AS C
ON T.`id` = C.`trans_id`;

查找每个月和每个国家/地区的已批准交易的数量及其总金额 需要对左表的月份和州进行分组
退单的数量及其总金额 则需要对右表的月份和州进行分组
连接后查询好像做不到

查看别人的方法
发现是分别查询然后用union合并

先查询已批准的订单

SELECT DATE_FORMAT(T.`trans_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`
FROM Transactions AS T
WHERE T.`state` = 'approved';

再查询退单的订单 字段需与上表保持一致

SELECT DATE_FORMAT(C.`charge_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`
FROM Transactions AS T
JOIN Chargebacks AS C
ON T.`id` = C.`trans_id`;

两表进行合并 为了区分 引入一个tag
且要用 union all避免重复的去掉

(SELECT DATE_FORMAT(T.`trans_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`, (SELECT 1) AS tag
FROM Transactions AS T
WHERE T.`state` = 'approved')
UNION ALL
(SELECT DATE_FORMAT(C.`charge_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`,(SELECT 0) AS tag
FROM Transactions AS T
JOIN Chargebacks AS C
ON T.`id` = C.`trans_id`);

之后与每月交易I 一致 按月份 国家分组 得到想要的计算结果即可

SELECT tmp.month, tmp.country,
SUM(tmp.tag) AS approved_count,
SUM(IF(tmp.tag = 1, tmp.amount, 0)) AS approved_amount,
COUNT(IF(tmp.tag = 0, 1, NULL)) AS chargeback_count,
SUM(IF(tmp.tag = 0, tmp.amount, 0)) AS chargeback_amount 
FROM ((SELECT DATE_FORMAT(T.`trans_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`, (SELECT 1) AS tag
FROM Transactions AS T
WHERE T.`state` = 'approved')
UNION ALL
(SELECT DATE_FORMAT(C.`charge_date`, '%Y-%m') AS MONTH, T.`country`, T.`state`, T.`amount`,(SELECT 0) AS tag
FROM Transactions AS T
JOIN Chargebacks AS C
ON T.`id` = C.`trans_id`)) AS tmp
GROUP BY tmp.month, tmp.country;