BEGIN
    -- If the delimiter passed was null, use an empty string.
    set @Delim = IsNull(@Delim, '')

    -- If the list is empty, set it equal to the value that was passed in.
    if (@ValueList IS NULL)
    begin
        set @ValueList = @Item
    end
    -- If the item passed in was not null, append it to the list.
    else if (@Item IS NOT NULL)
    begin
        set @ValueList = @Item + @Delim + @ValueList
    end

    RETURN @ValueList
END

select @Item + ISNULL(IsNull(@Delim,'') + @ValueList,'')

DECLARE @Str varchar(100) = 'FN,SP,TBL,091.889'

DECLARE @Separator VARCHAR(10) = ','

DECLARE @ValueAdd VARCHAR(10)  = '80990'


DECLARE @Action VARCHAR(10)  = 'R'--1. P..PostFix  2. R..Prefix

SELECT @Str = 
STUFF((
	SELECT 
	
	CASE WHEN @Action = 'R' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END
	+
	ISNULL(@Separator+@Str,'')
	+
	CASE WHEN @Action = 'P' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END
),1,1,'')

select @Str

DECLARE @Numbertable table
(
	ID INT PRIMARY KEY
	
)


INSERT INTO @Numbertable
select TOP 1000 ROW_NUMBER() OVER(order by si.object_id)
from sys.objects si,sys.objects s

DECLARE @separatolog TABLE
(
	SeparatorLogID INT,
	ID INT IDENTITY(1,1) PRIMARY KEY
)
DECLARE @Str varchar(100) = 'FN,SP,TBL,091.8891'

DECLARE @Separator VARCHAR(10) = ','

DECLARE @ValueAdd VARCHAR(10)  = '80990'


DECLARE @Action VARCHAR(10)  = '3'--1. P..PostFix  2. R..Prefix 3.Number for postion.

IF (ISNUMERIC(@Action) =0) 

BEGIN
SELECT @Str = 
STUFF((
        SELECT 
        
        CASE WHEN @Action = 'R' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END
        +
        ISNULL(@Separator+@Str,'')
        +
        CASE WHEN @Action = 'P' THEN ISNULL(@Separator+@ValueAdd,'') ELSE '' END
),1,1,'')

END
ELSE
BEGIN

INSERT INTO @separatolog

select distinct CHARINDEX(',',@str,ID)
from @Numbertable
where ID <= LEN(@str) and CHARINDEX(',',@str,ID) > 0
ORDER BY 1

DECLARE @CurrentSepartorID INT 

SET @CurrentSepartorID = isnull((select  SeparatorLogID
from @separatolog where ID = CAST(@Action AS INT)-1),CASE WHEN @Action = 1 THEN 0 ELSE LEN(@str)+1 END)


select @Str = STUFF(SUBSTRING(ISNULL(@Separator+@str,''),1,@CurrentSepartorID)
			  +ISNULL(@Separator+@ValueAdd,'')
			  +ISNULL(@Separator+NULLIF(SUBSTRING(@str,@CurrentSepartorID+1,LEN(@str)),''),''),1,1,'')
END

select @Str