How to use Update in Case statement with columns as aliases

Question

I have a table called sales_data with 3 columns (id int, udf varchar(20), date_of_sale datetime). I am trying to find the weekday of date_of_sale column by adjusting the time to 6 hours, now I have to update the udf column to the weekday corresponding to date_of_sale. I have an idea for a select query, but how do I update the udf column?

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

Add generated columns (https://dev.mysql.com/doc/refman/8.0/en/create-table-generated-columns.html) to simplify processing , and avoid data inconsistencies:

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 );

View the demo: https://dbfiddle.uk/2d5iIvBv

(I don't get the same weekdays, maybe it's the locale?)

Answer 2

Your above query is almost complete. You just need to add update statements.

The query below should work for you.

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;