MySQL use date() in CASE statement - mysql

Question

I am trying to use the date() function in MySQL to use only the date from a datetime format to create cases for total number of orders based on day of the week. However, when I run this query, I get a value of '0' for each case.
select
(CASE datetime WHEN date(datetime) = '2020-06-22' THEN count(order_id) ELSE 0 END) AS 'Mon'
,(CASE datetime WHEN date(datetime) = '2020-06-23' THEN count(order_id) ELSE 0 END) AS 'Tue'
,(CASE datetime WHEN date(datetime) = '2020-06-24' THEN count(order_id) ELSE 0 END) AS 'Wed'
,(CASE datetime WHEN date(datetime) = '2020-06-25' THEN count(order_id) ELSE 0 END) AS 'Thu'
,(CASE datetime WHEN date(datetime) = '2020-06-26' THEN count(order_id) ELSE 0 END) AS 'Fri'
from table;
I confirmed I do get my desired ouput for an individual date when I run the following query.
select count(order_id)
from table
where date(datetime) = '2020-06-22';

Answer 1

tray a query like this:
select
sum( if( weekday(datetime) = 0,1,0)) as 'Mon',
sum( if( weekday(datetime) = 1,1,0)) as 'Tue',
sum( if( weekday(datetime) = 2,1,0)) as 'Wed',
sum( if( weekday(datetime) = 3,1,0)) as 'Thu',
sum( if( weekday(datetime) = 4,1,0)) as 'Fri',
sum( if( weekday(datetime) = 5,1,0)) as 'Sat',
sum( if( weekday(datetime) = 6,1,0)) as 'Sun'
;
and you can use a
WHERE datetime BETWEEN '2020-06-22' AND '2020-06-26'
if you want to get a range of dates

Answer 2

MySQL has a nice feature where boolean values are treated as numbers, with 1 for true and 0 for false. So you can express the logic as:
select sum( weekday(datetime) = 0 ) as Mon,
sum( weekday(datetime) = 1 ) as Tue,
sum( weekday(datetime) = 2 ) as Wed,
sum( weekday(datetime) = 3 ) as Thu,
sum( weekday(datetime) = 4 ) as Fri,
sum( weekday(datetime) = 5 ) as Sat,
sum( weekday(datetime) = 6 ) as Sun
Or:
select sum( date(datetime) = '2020-06-22' ) as Mon,
sum( date(datetime) = '2020-06-23' ) as Tue,
sum( date(datetime) = '2020-06-24' ) as Wed,
sum( date(datetime) = '2020-06-25' ) as Thu,
sum( date(datetime) = '2020-06-26' ) as Fri,
sum( date(datetime) = '2020-06-27' ) as Sat,
sum( date(datetime) = '2020-06-28' ) as Sun
Your query doesn't work because it is an aggregation query (the COUNT()) but you have unaggregated columns in the SELECT. MySQL should be returning a parsing error -- and the most recent version does.
I also strongly discourage you from using single quotes to delimit columns. Here are three reasons:
Your column names are fine and don't need to be escaped.
Single quotes should only be used for string and date constants.
Backticks can be used if the identifiers do need to be escaped.