如何使用 Update in Case 語句和列作為別名

Question

我有一個名為 sales_data 的表，其中有 3 個欄位（id int、udf varchar(20)、date_of_sale datetime）。 我試圖透過將時間調整為 6 小時來尋找 date_of_sale 列的工作日，現在我必須將 udf 欄位更新為與 date_of_sale 對應的工作日。我有一個選擇查詢的想法，但如何更新 udf 列？

select weekday(subtime(s.date_of_sale ,'6:0:0')) as putdata,
CASE
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=0 THEN 'Sunday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=1 THEN 'Monday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=2 THEN 'Tuesday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=3 THEN 'Wednesday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=4 THEN 'Thursday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=5 THEN 'Friday'
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=6 THEN 'Saturday'
END as udf
from sales_data s;

Answer 1

新增產生欄位（https://dev.mysql.com/doc/refman/8.0/en/create-table- generated-columns.html）以簡化處理，並避免數據不一致：

alter table sales_data add weekday varchar(10) GENERATED ALWAYS
  AS (CASE   
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=0 THEN 'Sunday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=1 THEN 'Monday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=2 THEN 'Tuesday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=3 THEN 'Wednesday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=4 THEN 'Thursday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=5 THEN 'Friday'  
    WHEN weekday(subtime(s.date_of_sale ,'6:0:0'))=6 THEN 'Saturday'  
END );

查看示範：https://dbfiddle.uk/2d5iIvBv

#（我沒有得到相同的工作日，也許是區域設定？）

Answer 2

您上述的查詢已經差不多了。您只需要新增更新語句即可。

下面的查詢應該適合您。

update sales_data 
set udf = case   
    when weekday(subtime(date_of_sale,'6:0:0')) = 0 then 'Sunday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 1 then 'Monday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 2 then 'Tuesday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 3 then 'Wednesday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 4 then 'Thursday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 5 then 'Friday'  
    when weekday(subtime(date_of_sale,'6:0:0')) = 6 then 'Saturday'  
 end;