每日一题-19（周内每天的销售情况）

题19

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。

其中：

• Orders表：order_id, item_id 是该表主键
• Items表：item_id是该表主键,item_name是商品的名字,item_category是商品的类别.

解题思路：

• 先理解：DAYOFWEEK-返回一个介于1和7之间的数字，该数字对应于指定日期表达式是一周中的星期几。例如，1对应星期日，2对应星期一，……，7对应星期六;
• 用case when建立新的列，再往里面填充内容

select
    a.item_category as CATEGORY,
    sum(case when weekday(order_date)=0 then quantity else 0 end) as MONDAY,
    sum(case when weekday(order_date)=1 then quantity else 0 end) as TUESDAY,
    sum(case when weekday(order_date)=2 then quantity else 0 end) as WEDNESDAY,
    sum(case when weekday(order_date)=3 then quantity else 0 end) as THURSDAY,
    sum(case when weekday(order_date)=4 then quantity else 0 end) as FRIDAY,
    sum(case when weekday(order_date)=5 then quantity else 0 end) as SATURDAY,
    sum(case when weekday(order_date)=6 then quantity else 0 end) as SUNDAY    
from items a
left join orders b on
    a.item_id=b.item_id
group by CATEGORY
order by CATEGORY;