探讨如何计算age的sql函数

ALTER FUNCTION [dbo].[get_FullAge]
(
 @birthday datetime, @currentDay datetime
)
RETURNS INT
AS
BEGIN
DECLARE @age INT
SET @age = DATEDIFF(YEAR, @birthday, @currentDay)
IF DATEDIFF(DAY, DATEADD(YEAR, @age, @birthday), @currentDay) <= 0
 SET @age = @age - 1
IF DATEPART(MONTH, @birthday) = 2 AND DATEPART(DAY, @birthday) = 29 AND DATEPART(MONTH, @currentDay) = 3
 AND DATEPART(DAY, @currentDay) = 1 AND
 NOT (YEAR(@currentDay) % 4 = 0 AND (YEAR(@currentDay) % 100 !=0 OR YEAR(@currentDay) % 400 = 0))
 SET @age = @age - 1
IF @age < 0
 SET @age = 0
  RETURN @age
END