Concatenate Rows into String with Many to Many Joins

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