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DatePart, DateAdd and DateDiff functions in SQL Server - Part 27

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DatePart(DatePart, Date) - Returns an integer representing the specified DatePart. This function is simialar to DateName(). DateName() returns nvarchar, where as DatePart() returns an integer. The valid DatePart parameter values are shown below.




Examples:
Select DATEPART(weekday, '2012-08-30 19:45:31.793') -- returns 5
Select DATENAME(weekday'2012-08-30 19:45:31.793') -- returns Thursday

DATEADD (datepart, NumberToAdd, date) - Returns the DateTime, after adding specified NumberToAdd, to the datepart specified of the given date.

Examples:
Select DateAdd(DAY, 20, '2012-08-30 19:45:31.793'
-- Returns 2012-09-19 19:45:31.793
Select DateAdd(DAY, -20, '2012-08-30 19:45:31.793'
-- Returns 2012-08-10 19:45:31.793

DATEDIFF(datepart, startdate, enddate) - Returns the count of the specified datepart boundaries crossed between the specified startdate and enddate.

Examples:
Select DATEDIFF(MONTH'11/30/2005','01/31/2006') -- returns 2
Select DATEDIFF(DAY, '11/30/2005','01/31/2006') -- returns 62

Consider the emaployees table below.


Write a query to compute the age of a person, when the date of birth is given. The output should be as shown below.



CREATE FUNCTION fnComputeAge(@DOB DATETIME)
RETURNS NVARCHAR(50)
AS
BEGIN

DECLARE @tempdate DATETIME, @years INT, @months INT, @days INT
SELECT @tempdate = @DOB

SELECT @years = DATEDIFF(YEAR, @tempdate, GETDATE()) - CASE WHEN (MONTH(@DOB) > MONTH(GETDATE())) OR (MONTH(@DOB) = MONTH(GETDATE()) AND DAY(@DOB) > DAY(GETDATE())) THEN ELSE END
SELECT @tempdate = DATEADD(YEAR, @years, @tempdate)

SELECT @months = DATEDIFF(MONTH, @tempdate, GETDATE()) - CASE WHEN DAY(@DOB) > DAY(GETDATE()) THEN ELSE END
SELECT @tempdate = DATEADD(MONTH, @months, @tempdate)

SELECT @days = DATEDIFF(DAY, @tempdate, GETDATE())

DECLARE @Age NVARCHAR(50)
SET @Age = Cast(@years AS  NVARCHAR(4)) + ' Years ' + Cast(@months AS  NVARCHAR(2))+ ' Months ' +  Cast(@days AS  NVARCHAR(2))+ ' Days Old'
RETURN @Age

End

Using the function in a query to get the expected output along with the age of the person.
Select Id, Name, DateOfBirth, dbo.fnComputeAge(DateOfBirth) as Age from tblEmployees

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