Fill NULL weekend on 'NULL'

Question

I created DB with this data:

Market table:

ID| Name    | Location
1 | NYSE    | USA
2 | NASDAQ  | USA
3 | CME     | USA
4 | Euronext| Netherlands

Weekend table:

ID| DayOff
1 | 2013-11-28
2 | 2013-12-25
3 | 2014-01-01
4 | 2014-01-21
5 | 2014-02-18
6 | 2014-03-29
7 | 2014-05-27
8 | 2014-07-04
9 | 2014-09-02
10| NULL

m2m_market_weekends table:

market_id | weekend_id
1         | 1
1         | ... 
1         | 9
2         | 1
2         | ...
2         | 9
2         | 1
3         | ...
3         | 9
5         | 10

Write query, which groups data with equals IDs and append all dates at one string:

SELECT [Market].Name as [MarketName],
   IsNull(STUFF((SELECT '; ' + CAST([DayOff] AS nvarchar(15)) FROM [Weekend] FOR    XML PATH ('')),1, 1, ''), 'NULL') as [DayOff]
FROM [m2m_market_weekends]
INNER JOIN [Market]  ON [m2m_market_weekends].[market_id]  =  [Market].[ID]
INNER JOIN [Weekend] ON [m2m_market_weekends].[weekend_id] = [Weekend].[ID]
GROUP BY [Market].Name

And get:

MarketName | DayOff
NYSE       | 2013-11-28; ...
NASDAQ     | 2013-11-28; ...
CME        | 2013-11-28; ...
Euronext   | 2013-11-28; ...  <-- This market have NULL value at Weekend table

How fix this trouble, that this query returns at Euronext DayOff = 'NULL'?

Answer 1

You have no link from your correlated subquery to your main query (i.e. you will be bringing back a list of all weekends for all markets). This should work for you:

SELECT  MarketName = Market.Name,
        DayOff = ISNULL(STUFF(( SELECT  '; ' + CAST(DayOff AS VARCHAR(15)) 
                                FROM    m2m_market_weekends m2m
                                        INNER JOIN Weekend w
                                            ON m2m.weekend_id = w.ID
                                WHERE   m2m.market_id =  Market.ID
                                ORDER BY DayOff
                                FOR XML PATH (''), TYPE
                                ).value('.', 'NVARCHAR(MAX)'),
                        1, 1, ''),
                    'NULL')
FROM    Market
GROUP BY Market.Name, Market.ID;

Example on SQL Fiddle

Note I changed your XML method slightly since just using (SELECT .. FOR XML PATH('')) causes xml characters to be escaped (so if you had > inside the subquery it becomes >, however, if you use the 'value method, i.e. FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)) XML characters are preserved