Problem when getting the day name in SQL Server from a number - sql

Question

I am trying to get the dayname from SQL, with as input only a number representing a day of the week.
When I run this code
select
datename(dw, '20210115'),
datepart(dw, '20210115')
it returns Friday, 6
So I thought that the number 6 means Friday
Then I do the following
select datename(dw, 6)
but instead of returning Friday it returns Sunday
So, what is the proper way to get the dayname from SQL Server with only a weekday number as input?

Answer 1

The DateName() and DatePart() rely on specific settings (e.g. SET DATEFIRST)
So whilst a value of 6 might represent "Friday" for you right now, the same code might return a different value in a different session for you or for other users.
This is why I don't use those functions, but instead create a calendar table with metadata to inform me of the week day.
The code I use looks like this:
...
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 0 THEN 1 ELSE 0 END As is_monday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 1 THEN 1 ELSE 0 END As is_tuesday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 2 THEN 1 ELSE 0 END As is_wednesday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 3 THEN 1 ELSE 0 END As is_thursday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 4 THEN 1 ELSE 0 END As is_friday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 5 THEN 1 ELSE 0 END As is_saturday
, CASE WHEN DateDiff(dd, '1900-01-01', the_date) % 7 = 6 THEN 1 ELSE 0 END As is_sunday
...
This will work on all SQL Server databases, regardless of settings.
If you want this to just return the date name then it could be written as:
CASE DateDiff(dd, '1900-01-01', the_date) % 7
WHEN 0 THEN 'Monday'
WHEN 1 THEN 'Tuesday'
WHEN 2 THEN 'Wednesday'
WHEN 3 THEN 'Thursday'
WHEN 4 THEN 'Friday'
WHEN 5 THEN 'Saturday'
WHEN 6 THEN 'Sunday'
END AS day_of_week
How This Approach Works
We calculate the number of days between the input (in these examples the_date) and a known, fixed point in time.
The date 1900-01-01 is used because it is the "default" (not the right word, but I can't think of a better one right now!) i.e. what a value of 0 would be cast to.
1900-01-01 also happens to be a Monday!
Every 7th day after that is another Monday, which is what the modulus 7 (% 7) bit is responsible for working out!