convert datetime 127 和 Stuff 一起使用时，需要慎重注意删除字符的start position

Convert Datetime to 127 的格式是 yyyy-mm-ddThh:mi:ss.mmm

MSDN给出一个注意事项：

When the value for milliseconds (mmm) is 0, the milliseconds value is not displayed. For example, the value ‘2012-11-07T18:26:20.000 is displayed as ‘2012-11-07T18:26:20‘.

示例

declare @dt1 datetime
declare @dt2 datetime

set @dt1=‘2015-01-02 01:23:45.678‘
set @dt2=‘2015-01-02 01:23:45.000‘

select CONVERT(VARCHAR(50),@dt1, 127),len(CONVERT(VARCHAR(50),@dt1, 127)),
        CONVERT(VARCHAR(50),@dt2, 127),len(CONVERT(VARCHAR(50),@dt2, 127))

如果 stuff( convert(varchar(50),@datetime,127),20,4) 会出现一个“意外”的结果

declare @dt1 datetime
declare @dt2 datetime

set @dt1=‘2015-01-02 01:23:45.678‘
set @dt2=‘2015-01-02 01:23:45.000‘

select CONVERT(VARCHAR(50),@dt1, 127),len(CONVERT(VARCHAR(50),@dt1, 127)),
        CONVERT(VARCHAR(50),@dt2, 127),len(CONVERT(VARCHAR(50),@dt2, 127)),
        STUFF(CONVERT(VARCHAR(50),min(@dt1), 127),20,4,‘‘) ,
        STUFF(CONVERT(VARCHAR(50),min(@dt2), 127),20,4,‘‘) 

原因是Stuff函数返回null，MSDN给出解释

STUFF ( character_expression , start , length , replaceWith_expression )

If the start position or the length is negative, or if the starting position is larger than length of the first string, a null string is returned. If the start position is 0, a null value is returned. If the length to delete is longer than the first string, it is deleted to the first character in the first string.