题目
书籍表 Books：
+----------------+---------+
| Column Name | Type |
+----------------+---------+
| book_id | int |
| name | varchar |
| available_from | date |
+----------------+---------+
book_id 是这个表的主键。
订单表 Orders：
+----------------+---------+
| Column Name | Type |
+----------------+---------+
| order_id | int |
| book_id | int |
| quantity | int |
| dispatch_date | date |
+----------------+---------+
order_id 是这个表的主键。
book_id 是 Books 表的外键。
你需要写一段 SQL 命令，筛选出过去一年中订单总量 少于10本 的 书籍 。
注意：不考虑 上架（available from）距今 不满一个月 的书籍。并且 假设今天是 2019-06-23 。
下面是样例输出结果：
Books 表：
+---------+--------------------+----------------+
| book_id | name | available_from |
+---------+--------------------+----------------+
| 1 | "Kalila And Demna" | 2010-01-01 |
| 2 | "28 Letters" | 2012-05-12 |
| 3 | "The Hobbit" | 2019-06-10 |
| 4 | "13 Reasons Why" | 2019-06-01 |
| 5 | "The Hunger Games" | 2008-09-21 |
+---------+--------------------+----------------+
Orders 表：
+----------+---------+----------+---------------+
| order_id | book_id | quantity | dispatch_date |
+----------+---------+----------+---------------+
| 1 | 1 | 2 | 2018-07-26 |
| 2 | 1 | 1 | 2018-11-05 |
| 3 | 3 | 8 | 2019-06-11 |
| 4 | 4 | 6 | 2019-06-05 |
| 5 | 4 | 5 | 2019-06-20 |
| 6 | 5 | 9 | 2009-02-02 |
| 7 | 5 | 8 | 2010-04-13 |
+----------+---------+----------+---------------+
Result 表：
+-----------+--------------------+
| book_id | name |
+-----------+--------------------+
| 1 | "Kalila And Demna" |
| 2 | "28 Letters" |
| 5 | "The Hunger Games" |
+-----------+--------------------+
生成数据
CREATE TABLE Books(
book_id INT,
NAME VARCHAR(20),
available_from DATE,
PRIMARY KEY(book_id));
CREATE TABLE Orders3(
order_id INT,
book_id INT,
quantity INT,
dispatch_date DATE,
PRIMARY KEY(order_id));
INSERT INTO Books VALUE(1, "Kalila And Demna", '2010-01-01'),(2, "28 Letters", '2012-05-12 '),
(3, "The Hobbit", '2019-06-10'),(4, "13 Reasons Why", '2019-06-01'),
(5, "The Hunger Games", '2008-09-21');
INSERT INTO Orders3 VALUE(1, 1, 2, '2018-07-26'),(2, 1, 1, '2018-11-05'),
(3, 3, 8, '2019-06-11'),(4, 4, 6, '2019-06-05'),
(5, 4, 5, '2019-06-20'),(6, 5, 9, '2009-02-02'),
(7, 5, 8, '2010-04-13');
解答
筛选出过去一年中订单总量 少于10本 的 书籍 。
注意：不考虑 上架（available from）距今 不满一个月 的书籍。并且 假设今天是 2019-06-23
先去掉上架（available from）距今 不满一个月 的书籍
SELECT *
FROM Books AS B
WHERE B.available_from < '2019-05-23';
再选出过去一年的订单
SELECT O.`book_id`, O.`quantity`
FROM orders3 AS O
WHERE O.`dispatch_date` BETWEEN '2018-06-23' AND '2019-06-23';
两表连接
SELECT *
FROM (SELECT B.`book_id`, B.`name`
FROM Books AS B
WHERE B.available_from < '2019-05-23') tmp1
LEFT JOIN (SELECT O.`book_id`, O.`quantity`
FROM orders3 AS O
WHERE O.`dispatch_date` BETWEEN '2018-06-23' AND '2019-06-23') tmp2
ON tmp1.book_id = tmp2.book_id
分组求和
SELECT tmp1.book_id, tmp1.name, SUM(tmp2.quantity)
FROM (SELECT *
FROM Books AS B
WHERE B.available_from < '2019-05-23') tmp1
LEFT JOIN (SELECT *
FROM orders3 AS O
WHERE O.`dispatch_date` BETWEEN '2018-06-23' AND '2019-06-23') tmp2
ON tmp1.book_id = tmp2.book_id
GROUP BY tmp1.book_id;
这里的NULL需要注意 其实是0的意思 后边的判断需要注意或者这里用IFNULL进行处理
SELECT tmp1.book_id, tmp1.name
FROM (SELECT *
FROM Books AS B
WHERE B.available_from < '2019-05-23') tmp1
LEFT JOIN (SELECT *
FROM orders3 AS O
WHERE O.`dispatch_date` BETWEEN '2018-06-23' AND '2019-06-23') tmp2
ON tmp1.book_id = tmp2.book_id
GROUP BY tmp1.book_id
HAVING (SUM(tmp2.quantity) < 10) OR (SUM(tmp2.quantity) IS NULL);