Question

All three statements below return the same result. So I’m struggling to see what the difference is between using dayofyear, day and weekday. Am I missing something or are they all equivalent?

   SELECT DATEADD(dayofyear,1,'20111231')
   SELECT DATEADD(day,1,'20111231')
   SELECT DATEADD(weekday,1,'20111231')

They all return 2012-01-01 00:00:00.000

Answer 1

In that use, no they return the same. Though “day” feels natural for this use.

The difference comes in (e.g.) DATEPART:

SELECT DATEPART(dayofyear,'20111231') -- 365
SELECT DATEPART(day,'20111231') -- 31
SELECT DATEPART(weekday,'20111231') --7

Here’s the MSDN ref on it:

dayofyear , day, and weekday return
the same value.