Invalid length parameter passed to the LEFT or SUBSTRING function - sql

Question

Following is my query and it works fine when I have like TRAILER_MAKE_MODEL 'testing ~ testing, test ~ testq,' ends with comma but can't handle 'testing ~ testing, test ~ testq' same as for other variable TRAILER_IDV. I tried my best but cant work it out any help would be appreciated.
My aim is to get the comma separated value for xml.
DECLARE #TRAILER_MAKE_MODEL VARCHAR(MAX)
DECLARE #TRAILER_IDV VARCHAR(MAX)
DECLARE #USER_TYPE VARCHAR(MAX)
DECLARE #START_INDEX_1 INT
DECLARE #START_INDEX_4 INT
DECLARE #END_INDEX_1 INT
DECLARE #END_INDEX_4 INT
DECLARE #VALUE VARCHAR(50)
DECLARE #QUERY VARCHAR(MAX)
set #TRAILER_MAKE_MODEL='testing ~ testing, test ~ testq,'
set #TRAILER_IDV='3500, 3400,'
set #USER_TYPE='MOBILE'
set #QUERY = ''
set #START_INDEX_1 = 1
set #START_INDEX_4 = 1
set #END_INDEX_1 = 0
if ISNULL(#TRAILER_MAKE_MODEL,'') <> ''
begin
WHILE #START_INDEX_1 > 0 and #START_INDEX_1 < len(#TRAILER_MAKE_MODEL)
BEGIN
SET #END_INDEX_1 = CHARINDEX(',',#TRAILER_MAKE_MODEL,#START_INDEX_1)
if #END_INDEX_1 = 0 and #START_INDEX_1 = 1
Begin
SET #END_INDEX_1 = len(#TRAILER_MAKE_MODEL)
END
if #USER_TYPE <> 'MOBILE'
Begin
SET #END_INDEX_1 = #END_INDEX_1 +1
End
SET #VALUE = SUBSTRING(#TRAILER_MAKE_MODEL,#START_INDEX_1,#END_INDEX_1 - #START_INDEX_1)
SET #QUERY = #QUERY + 'UNION ALL SELECT ''' + #VALUE + ''' TRAILER_MAKE_MODEL'
SET #END_INDEX_4 = CHARINDEX(',',#TRAILER_IDV,#START_INDEX_4)
if #END_INDEX_4 = 0 and #START_INDEX_4 = 1
Begin
SET #END_INDEX_4 = len(#TRAILER_IDV)
END
if #USER_TYPE <> 'MOBILE'
Begin
SET #END_INDEX_4 = #END_INDEX_4 +1
End
SET #VALUE = SUBSTRING(#TRAILER_IDV,#START_INDEX_4,#END_INDEX_4 - #START_INDEX_4)
SET #QUERY = #QUERY + ',' + #VALUE + 'TRAILER_IDV '
print #QUERY
SET #START_INDEX_1 = #END_INDEX_1 + 1
SET #START_INDEX_4 = #END_INDEX_4 + 1
END
select #QUERY=substring(#QUERY, 10, LEN(#QUERY) - 9)
EXEC (#QUERY)
END

Answer 1

You already have a lot of code here so two extra lines where you assign a comma at the end of each string should probably not slow things down for you or make the code less maintainable.
SET #TRAILER_MAKE_MODEL += ',';
SET #TRAILER_IDV += ',';
I don't really understand what your code does but to get the result you are getting you can use a split string function that returns the index of the item like this.
select T1.Item as TRAILER_MAKE_MODEL,
T2.Item as TRAILER_IDV
from dbo.SplitString(#TRAILER_MAKE_MODEL, ',') as T1
inner join dbo.SplitString(#TRAILER_IDV, ',') as T2
on T1.ItemNumber = T2.ItemNumber