SQL: Concatenate column values in a single row into a string separated by comma

Question

Let's say I have a table like this in SQL Server:

Id    City           Province             Country
1     Vancouver      British Columbia     Canada
2     New York       null                 null
3     null           Adama                null
4     null           null                 France
5     Winnepeg       Manitoba             null
6     null           Quebec               Canada
7     Seattle        null                 USA 

How can I get a query result so that the location is a concatenation of the City, Province, and Country separated by ", ", with nulls omitted. I'd like to ensure that there aren't any trailing comma, preceding commas, or empty strings. For example:

Id    Location
1     Vancouver, British Columbia, Canada
2     New York
3     Adama
4     France
5     Winnepeg, Manitoba
6     Quebec, Canada
7     Seattle, USA

Answer 1

I think this takes care of all of the issues I spotted in other answers. No need to test the length of the output or check if the leading character is a comma, no worry about concatenating non-string types, no significant increase in complexity when other columns (e.g. Postal Code) are inevitably added...

DECLARE @x TABLE(Id INT, City VARCHAR(32), Province VARCHAR(32), Country VARCHAR(32));

INSERT @x(Id, City, Province, Country) VALUES
(1,'Vancouver','British Columbia','Canada'),
(2,'New York' , null             , null   ),
(3, null      ,'Adama'           , null   ),
(4, null      , null             ,'France'),
(5,'Winnepeg' ,'Manitoba'        , null   ),
(6, null      ,'Quebec'          ,'Canada'),
(7,'Seattle'  , null             ,'USA'   );

SELECT Id, Location = STUFF(
      COALESCE(', ' + RTRIM(City),     '') 
    + COALESCE(', ' + RTRIM(Province), '') 
    + COALESCE(', ' + RTRIM(Country),  '')
    , 1, 2, '')
  FROM @x;

SQL Server 2012 added a new T-SQL function called CONCAT, but it is not useful here, since you still have to optionally include commas between discovered values, and there is no facility to do that - it just munges values together with no option for a separator. This avoids having to worry about non-string types, but doesn't allow you to handle nulls vs. non-nulls very elegantly.