每周的第一天日期

获取某周的第一天日期。
参考下面MS SQL Server代码：

CREATE FUNCTION [dbo].[svf_StartOfWeek]
(    
    @Year INT,
    @WK INT
)
RETURNS DATETIME
AS
BEGIN        
    DECLARE @FirstDayOfYear DATETIME = DATEFROMPARTS(@Year, 1, 1);
    DECLARE @IsFirstDayInFirstWeek INT = CASE WHEN (DATEPART(WEEKDAY, @FirstDayOfYear) <= 4) THEN 1 ELSE 0 END;     
    RETURN DATEADD(WEEK, @WK - @IsFirstDayInFirstWeek, DATEADD(DAY, (1 - DATEPART(WEEKDAY, @FirstDayOfYear)), @FirstDayOfYear));
END

 
附加：获取某周最后一天：

CREATE FUNCTION [dbo].[svf_EndOfWeek]
(    
    @Year INT,
    @WK INT
)
RETURNS DATETIME
AS
BEGIN        
    DECLARE @FirstDayOfYear DATETIME = DATEFROMPARTS(@Year, 1, 1);
    DECLARE @IsFirstDayInFirstWeek INT = CASE WHEN (DATEPART(WEEKDAY, @FirstDayOfYear) <= 4) THEN 1 ELSE 0 END;     
    RETURN  DATEADD(WEEK, @WK - @IsFirstDayInFirstWeek, DATEADD(DAY, (@@DATEFIRST - DATEPART(WEEKDAY, @FirstDayOfYear)), @FirstDayOfYear));
END
GO

以上2个函数，受到语言环境和@@DATEFIRST值的影响。