CREATE FUNCTION fGetStrBySplit
(
@Source VARCHAR(max),
@Index INT,
@SplitChar VARCHAR(1)
)
RETURNS varchar(MAX)
AS
BEGIN

DECLARE @Len INT
DECLARE @n INT = 0
DECLARE @ChIndex INT
DECLARE @Result VARCHAR(MAX)
--获取总长度
SET @Len = LEN(@Source)
--获取切割字符位置
SET @ChIndex = CHARINDEX(@SplitChar,@Source)

WHILE @ChIndex > 0
BEGIN
IF(@n = @Index)
BEGIN
SET @Source = SUBSTRING(@Source,0,@ChIndex)
BREAK
END

SET @Source = SUBSTRING(@Source,@ChIndex+1,@Len)
SET @ChIndex = CHARINDEX(@SplitChar,@Source)

SET @Len = LEN(@Source)
SET @n = @n + 1

END

RETURN @Source
END
GO

--调用
DECLARE @value VARCHAR(max)
SET @value = dbo.fGetStrBySplit('645002*01_45854_183677_12',0,'_')
PRINT @value