Convert day of year to datetime in SQL Server

问题

In a database table (SQL Server 2012), I have dates saved in the yyyyxxx format, where xxx is the day of the year. For example 2015-08-11 is 2015223.

How can I efficiently convert this format to DATETIME? How about converting DATETIME to this format?

回答1:

Get the Year part, convert to datetime + add days

DECLARE @var NVARCHAR(100) = '2015223';

SELECT CAST(LEFT(@var, 4) AS DATETIME) +  CAST(RIGHT(@var, 3) AS INT) - 1;

Or:

DECLARE @var NVARCHAR(100) = '2015223';

SELECT DATEADD(dd,CAST(RIGHT(@var, 3) AS INT)-1, CAST(LEFT(@var, 4) AS DATETIME))

Or if yyyyxxx is INT:

DECLARE @var INT = 2015223;

SELECT DATEADD(dd,(@var%1000)-1, CAST(CAST((@var/1000) AS NVARCHAR(100)) 
               AS DATETIME));

LiveDemo

回答2:

Since there is a good answer for the first question, let's talk about the second one.

How about converting DATETIME to this format?

DATENAME function can help to solve it. It's gonna look like that:

SELECT DATENAME(year, @var) + RIGHT('00' + DATENAME(dayofyear, @var), 3)

来源：https://stackoverflow.com/questions/32331704/convert-day-of-year-to-datetime-in-sql-server