1. 题目
表：Orders
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| order_id | int |
| customer_id | int |
| order_date | date |
| item_id | varchar |
| quantity | int |
+---------------+---------+
(order_id, item_id) 是该表主键
该表包含了订单信息
order_date 是id为 item_id 的商品被id为 customer_id 的消费者订购的日期.
表：Items
+---------------------+---------+
| Column Name | Type |
+---------------------+---------+
| item_id | varchar |
| item_name | varchar |
| item_category | varchar |
+---------------------+---------+
item_id 是该表主键
item_name 是商品的名字
item_category 是商品的类别
你是企业主，想要获得分类商品和周内每天的销售报告。
写一个SQL语句，报告 周内每天 每个商品类别下订购了多少单位。
返回结果表单 按商品类别排序 。
查询结果格式如下例所示：
Orders 表：
+------------+--------------+-------------+--------------+-------------+
| order_id | customer_id | order_date | item_id | quantity |
+------------+--------------+-------------+--------------+-------------+
| 1 | 1 | 2020-06-01 | 1 | 10 |
| 2 | 1 | 2020-06-08 | 2 | 10 |
| 3 | 2 | 2020-06-02 | 1 | 5 |
| 4 | 3 | 2020-06-03 | 3 | 5 |
| 5 | 4 | 2020-06-04 | 4 | 1 |
| 6 | 4 | 2020-06-05 | 5 | 5 |
| 7 | 5 | 2020-06-05 | 1 | 10 |
| 8 | 5 | 2020-06-14 | 4 | 5 |
| 9 | 5 | 2020-06-21 | 3 | 5 |
+------------+--------------+-------------+--------------+-------------+
Items 表：
+------------+----------------+---------------+
| item_id | item_name | item_category |
+------------+----------------+---------------+
| 1 | LC Alg. Book | Book |
| 2 | LC DB. Book | Book |
| 3 | LC SmarthPhone | Phone |
| 4 | LC Phone 2020 | Phone |
| 5 | LC SmartGlass | Glasses |
| 6 | LC T-Shirt XL | T-Shirt |
+------------+----------------+---------------+
Result 表：
+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| Category | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday |
+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| Book | 20 | 5 | 0 | 0 | 10 | 0 | 0 |
| Glasses | 0 | 0 | 0 | 0 | 5 | 0 | 0 |
| Phone | 0 | 0 | 5 | 1 | 0 | 0 | 10 |
| T-Shirt | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
在周一(2020-06-01, 2020-06-08)，Book分类(ids: 1, 2)下，总共销售了20个单位(10 + 10)
在周二(2020-06-02)，Book分类(ids: 1, 2)下，总共销售了5个单位
在周三(2020-06-03)，Phone分类(ids: 3, 4)下，总共销售了5个单位
在周四(2020-06-04)，Phone分类(ids: 3, 4)下，总共销售了1个单位
在周五(2020-06-05)，Book分类(ids: 1, 2)下，总共销售了10个单位，Glasses分类(ids: 5)下，总共销售了5个单位
在周六, 没有商品销售
在周天(2020-06-14, 2020-06-21)，Phone分类(ids: 3, 4)下，总共销售了10个单位(5 + 5)
没有销售 T-Shirt 类别的商品
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/sales-by-day-of-the-week
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
2. 解题
Mysql的日期转换成星期[某天对应周几]
dayname 函数返回英文星期几，dayofweek返回第几天，周天是1
date_format + %w，周天返回 0，%W，返回英文星期几
select date_format("2020-7-26",'%W')
union
select date_format("2020-7-26",'%w')
{"headers": ["date_format(\"2020-7-26\",'%W')"],
"values": [
["Sunday"],
["0"]]}
# Write your MySQL query statement below
select item_category Category,
ifnull(sum(case when dayname(o.order_date)='Monday' then quantity else 0 end), 0) Monday,
ifnull(sum(case when dayname(o.order_date)='Tuesday' then quantity else 0 end), 0) Tuesday,
ifnull(sum(case when dayname(o.order_date)='Wednesday' then quantity else 0 end), 0) Wednesday,
ifnull(sum(case when dayname(o.order_date)='Thursday' then quantity else 0 end), 0) Thursday,
ifnull(sum(case when dayname(o.order_date)='Friday' then quantity else 0 end), 0) Friday,
ifnull(sum(case when dayname(o.order_date)='Saturday' then quantity else 0 end), 0) Saturday,
ifnull(sum(case when dayname(o.order_date)='Sunday' then quantity else 0 end), 0) Sunday
from Items i left join Orders o
using(item_id)
group by item_category
order by item_category
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