Concatenate Rows into String with Many to Many Joins - sql

Question

I have a query that is working for almost all my data scenarios...except one. I have a master table, a detail table (with one to many line items), and many description tables that join to the detail table (Code Descriptions, etc). I need to concatenate all detail records into one string value so that I have one record per master record. So if there's 3 details for the master record, all values need to be concatenated into one record to represent the master.
My issue comes when the Master record has multiple detail records but the detail records don't have a match to the description tables 1 to 1. For instance, in my scenario the master record has 3 detail records which only one of the detail records has a description record. So 1 to 3 to 1, Master to Detail to Description. This is causing an issue. When trying to concatenate the records the code is not working because of the NULL value created from the Detail to Description join. The only way I can seem to get this to work is to do a Distinct sub-query and then do my concatenation logic on the outside of that. I feel like there has to be a better way or that I am simply missing something. I provided example code below to show my issue. There are 3 selects that get run. The first is the flat result with all records from the joins. The second is my original logic showing the flaw. The third is a working version that I am hoping someone knows how to do better. I greatly appreciate any help with this issue.
DECLARE #Notification table(
SystemID int NOT NULL,
NotificationID int
);
DECLARE #NotificationItems table(
SystemID int NOT NULL,
NotificationID VARCHAR(100),
LineItem VARCHAR(100)
);
DECLARE #NotificationCauses table(
SystemID int NOT NULL,
NotificationID VARCHAR(100),
LineItem VARCHAR(100),
TestValue VARCHAR(100)
);
INSERT INTO #Notification
SELECT 40,1 UNION
SELECT 40,2 UNION
SELECT 40,3 UNION
SELECT 40,4
INSERT INTO #NotificationItems
SELECT 40,1,1 UNION
SELECT 40,1,2 UNION
SELECT 40,1,3 UNION
SELECT 40,2,1 UNION
SELECT 40,2,2 UNION
SELECT 40,3,1
INSERT INTO #NotificationCauses
SELECT 40,1,1,'Code_A' UNION
SELECT 40,2,1,'Code_B' UNION
SELECT 40,2,2,'Code_C' UNION
SELECT 40,3,1,'Code_D'
--SELECT *
--FROM #Notification
--SELECT *
--FROM #NotificationItems
SELECT *
FROM #Notification AS n
LEFT OUTER JOIN #NotificationItems AS ni
ON n.NotificationID = ni.NotificationID
AND n.SystemID = ni.SystemID
LEFT OUTER JOIN #NotificationCauses AS nc
ON ni.NotificationID = nc.NotificationID
AND ni.SystemID = nc.SystemID
AND ni.LineItem = nc.LineItem
SELECT DISTINCT n.SystemID, n.NotificationID
,SUBSTRING(
(
SELECT DISTINCT
CASE WHEN LTRIM(RTRIM(ni1.LineItem)) <> ISNULL('','') THEN ', '+ni1.LineItem ELSE '' END AS [text()]
FROM #NotificationItems AS ni1
WHERE ni1.SystemID = ni.SystemID AND ni1.NotificationID = ni.NotificationID --AND a1.LineItem = a.LineItem
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [LineItem]
,SUBSTRING(
(
SELECT DISTINCT
CASE WHEN LTRIM(RTRIM(nc1.TestValue)) <> ISNULL('','') THEN ', '+nc1.TestValue ELSE '' END AS [text()]
FROM #NotificationCauses AS nc1
WHERE nc1.SystemID = nc.SystemID AND nc1.NotificationID = nc.NotificationID --AND nc1.LineItem = nc.LineItem
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [TestValues]
FROM #Notification AS n
LEFT OUTER JOIN #NotificationItems AS ni
ON n.SystemID = ni.SystemID
AND n.NotificationID = ni.NotificationID
LEFT OUTER JOIN #NotificationCauses AS nc
ON ni.SystemID = nc.SystemID
AND ni.NotificationID = nc.NotificationID
AND ni.LineItem = nc.LineItem
SELECT DISTINCT SystemID, NotificationID
,SUBSTRING(
(
SELECT DISTINCT
CASE WHEN LTRIM(RTRIM(a1.LineItem)) <> ISNULL('','') THEN ', '+a1.LineItem ELSE '' END AS [text()]
FROM #NotificationItems AS a1
WHERE a1.SystemID = a.SystemID AND a1.NotificationID = a.NotificationID --AND a1.LineItem = a.LineItem
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [LineItem]
,SUBSTRING(
(
SELECT DISTINCT
CASE WHEN LTRIM(RTRIM(a1.TestValue)) <> ISNULL('','') THEN ', '+a1.TestValue ELSE '' END AS [text()]
FROM #NotificationCauses AS a1
WHERE a1.SystemID = a.SystemID AND a1.NotificationID = a.NotificationID --AND a1.LineItem = a.LineItem
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [TestValues]
FROM
(
SELECT DISTINCT n.NotificationID, n.SystemID, ni.LineItem, nc.TestValue
FROM #Notification AS n
LEFT OUTER JOIN #NotificationItems AS ni
ON n.SystemID = ni.SystemID
AND n.NotificationID = ni.NotificationID
LEFT OUTER JOIN #NotificationCauses AS nc
ON ni.SystemID = nc.SystemID
AND ni.NotificationID = nc.NotificationID
AND ni.LineItem = nc.LineItem
) AS a

Answer 1

I have cleared up the query and that's the result
SELECT n.SystemID, n.NotificationID
, SUBSTRING((SELECT COALESCE(', ' + ni.LineItem, '') [text()]
FROM NotificationItems AS ni
WHERE ni.SystemID = n.SystemID
AND ni.NotificationID = n.NotificationID
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [LineItem]
, SUBSTRING((SELECT COALESCE(', ' + nc.TestValue, '') [text()]
FROM NotificationItems AS ni
INNER JOIN NotificationCauses nc
ON ni.SystemID = nc.SystemID
AND ni.NotificationID = nc.NotificationID
AND ni.LineItem = nc.LineItem
WHERE ni.SystemID = n.SystemID
AND ni.NotificationID = n.NotificationID
ORDER BY 1
FOR XML PATH ('')
), 2, 1000) AS [TestValues]
FROM Notification n
Those are the "smell" founded:
the ISNULL('', ''), it really does nothing, replaced with blank string '' (more to come)
the FROM of the main select, there are three table when only one is really used, the unused two are removed, that needed an update of the FROM and WHERE condition subqueries
the DISTINCT of the subqueries, again it does nothing, stripped away
the CASE in the subqueries add a comma before a value, if the trimmed value is not null, that is exactly what COALESCE(', ' + value, '') does (if the trim is needed re-add it)
Everything else is only formatting the query my way, 'cause it's easier to read if it's formatted your way (something like review by refactoring)
Here is a SQLFiddle demo of the cleared query with the data provided