SQL Join Query Returning Null - sql

Question

I've got a query that references three tables.
The tables are as follows:
FORM_FACTOR
FORM_FACTOR_ID | FORM_FACTOR_DESCRIPTION
AOF_ORDER_LINE_QUEUE
SO_LINE_NUMBER | FORM_FACTOR_ID
AOF_ORDER_PARTS
SERIAL_NUMBER | SO_LINE_NUMBER
The idea is to get the FORM_FACTOR_DESCRIPTION using the SERIAL_NUMBER of the part. I wrote a query to do that, but for whatever reason it's returning null. I could swear this query was working, but apparently there is a flaw. Help is appreciated.
The query:
DECLARE #serial VARCHAR(50)
SET #serial = 'somestring'
SELECT
ff.[FORM_FACTOR_DESCRIPTION]
FROM
[dbo].[FORM_FACTOR] AS ff
JOIN
[dbo].[AOF_ORDER_LINE_QUEUE] AS lQ ON ff.[FORM_FACTOR_ID] = lQ.[FORM_FACTOR_ID]
JOIN
[dbo].[AOF_ORDER_PARTS] AS oO ON oO.[SO_LINE_NUMBER] = lQ.[SO_LINE_NUMBER]
WHERE
oO.[SERIAL_NUMBER] = #serial

Answer 1

Not really a solution here. It still remains an unsolved mystery at this point.
But to look for the root cause, perhaps you could turn the joins around and use left joins?
Perhaps it's then easier to spot at what point the joins return NULL on the right side.
In the example below I messed around with COLLATION.
Just ignore and remove it, most likely it has nothing to do with it.
DECLARE #serial VARCHAR(50);
SET #serial = 'EOXH2T100008' ;
SELECT DB_NAME() as current_db, DATABASEPROPERTYEX(DB_NAME(), 'Collation') as db_collation;
--#SQL_Latin1_General_CP1_CI_AS
--SELECT Name, Description FROM fn_helpcollations() where name like '%Latin1%' ;
declare #FORM_FACTOR table (FORM_FACTOR_ID int, FORM_FACTOR_DESCRIPTION varchar(30));
insert into #FORM_FACTOR (FORM_FACTOR_ID, FORM_FACTOR_DESCRIPTION) values (1,'test 1'),(2,'test 2');
declare #AOF_ORDER_LINE_QUEUE table (ORDER_LINE_QUEUE_ID int identity(1,1), FORM_FACTOR_ID int, SO_LINE_NUMBER int);
insert into #AOF_ORDER_LINE_QUEUE (FORM_FACTOR_ID, SO_LINE_NUMBER) values (1,100),(3,100),(2,200);
declare #AOF_ORDER_PARTS table (ORDER_PARTS_ID int identity(1,1), SO_LINE_NUMBER int, SERIAL_NUMBER VARCHAR(50) COLLATE SQL_Latin1_General_CP850_BIN);
insert into #AOF_ORDER_PARTS (SO_LINE_NUMBER, SERIAL_NUMBER) values (100,'EOXH2T100008'),(200,'eOXH2T100008');
SELECT
oO.ORDER_PARTS_ID, oO.SERIAL_NUMBER, oO.SO_LINE_NUMBER,
lQ.ORDER_LINE_QUEUE_ID, lQ.FORM_FACTOR_ID,
ff.FORM_FACTOR_DESCRIPTION
FROM #AOF_ORDER_PARTS AS oO
LEFT JOIN #AOF_ORDER_LINE_QUEUE AS lQ ON lQ.[SO_LINE_NUMBER] = oO.[SO_LINE_NUMBER]
LEFT JOIN #FORM_FACTOR AS ff ON ff.[FORM_FACTOR_ID] = lQ.[FORM_FACTOR_ID]
WHERE oO.[SERIAL_NUMBER] = #serial;