DATEPARTdayofyear

DATEPART(dayofyear) is a datepart function used in Microsoft SQL Server’s T-SQL to return the ordinal day of the year for a given date. It extracts the position of a date within its year, counting from January 1 as 1.

The typical syntax is DATEPART ( datepart , startdate ), where datepart is dayofyear (often written as dayofyear or its abbreviation in some contexts) and startdate is a date or datetime value. For example, SELECT DATEPART(dayofyear, '2023-02-15') returns 46, since February 15 is the 46th day of 2023. The result ranges from 1 to 365 in common years and 1 to 366 in leap years. Time components in the input are ignored for this calculation; only the date portion matters.

Common uses include calculating year-to-date metrics, sorting or grouping data by the position of a date within its year, or performing seasonal analyses that depend on the day number rather than the exact date. The function returns NULL if the input date is NULL.

Notes and caveats: SQL Server supports various datepart tokens; dayofyear is one of them, and other systems may use different names or equivalents. For example, PostgreSQL uses extract(doy from date), while Oracle might use to_char(date, 'DDD'). When writing portable SQL, rely on the specific dialect’s function to obtain the day-of-year.

In performance terms, DATEPART is a scalar function and can affect query plans if used on indexed columns in filtering conditions, so indexing strategy and query design should consider such usage.