USE [自己的数据库名]
GO
/****** Object:  UserDefinedFunction [dbo].[GetMonthDayNum]    Script Date: 2020/12/9 13:07:49 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
/*=========================================================
创建人：LJF
描  述：获取输入日期当月的天数。
测  试：SELECT [dbo].[GetMonthDayNum]('2020-11-01')
=========================================================*/
CREATE FUNCTION [dbo].[GetMonthDayNum]
(
    @when DATETIME
)
RETURNS INT
AS
BEGIN
    DECLARE @rv INT;

    IF DATEPART(MONTH, @when) = 2
    BEGIN
        IF DATEPART(YEAR, @when) % 400 = 0
            SELECT @rv = 29;
        ELSE IF DATEPART(YEAR, @when) % 100 = 0
            SELECT @rv = 28;
        ELSE IF DATEPART(YEAR, @when) % 4 = 0
            SELECT @rv = 29;
        ELSE
            SELECT @rv = 28;
    END; -- if
    ELSE
    BEGIN
        SELECT @rv = CASE (DATEPART(MONTH, @when))
                         WHEN 4 THEN
                             30
                         WHEN 6 THEN
                             30
                         WHEN 9 THEN
                             30
                         WHEN 11 THEN
                             30
                         ELSE
                             31
                     END; -- case
    END; -- else

    RETURN @rv;

END; -- fn def