TSQL: FOR XML PATH('') Failing To Group

Question

I'm trying to group column values by a specific column using FOR XML PATH('') in TSQL. This is the result in both cases (note that the without XML code - ie: SELECT * FROM @xml - is the same as the with XML code):

Class          |     Animals
=================================
Asteroidea     |     Starfish
Mammalia       |     Dog
Mammalia       |     Cat
Mammalia       |     Coyote
Reptilia       |     Crocodile
Reptilia       |     Lizard

According to this article and this article (note that the second article leaves out the GROUP BY, which I'm unsure how the author managed to pull this off without it - I've tried and it only generates all the values), the syntax should be as shown below this:

DECLARE @xml TABLE(
    Animal VARCHAR(50),
    Class VARCHAR(50)
)

INSERT INTO @xml
VALUES ('Dog','Mammalia')
    , ('Cat','Mammalia')
    , ('Coyote','Mammalia')
    , ('Starfish','Asteroidea')
    , ('Crocodile','Reptilia')
    , ('Lizard','Reptilia')

SELECT x1.Class
    , STUFF((SELECT ',' + x2.Animal AS [text()] 
    FROM @xml x2 
    WHERE x1.Animal = x2.Animal 
    ORDER BY x2.Animal 
    FOR XML PATH('')),1,1,'' ) AS "Animals"
FROM @xml x1
GROUP BY Class

After a few hours, between these examples and the above code, I fail to see where I'm wrong on syntax, but I'm receiving the error "Column '@xml.Animal' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause." Note that if I leave off the GROUP BY clause, it still doesn't produce the values in the appropriate manner. Another set of eyes would be useful.

Answer 1

I think you have your WHERE clause using the wrong column, you want to use Class not Animal:

SELECT x1.Class
    , STUFF((SELECT ',' + x2.Animal AS [text()] 
    FROM @xml x2 
    WHERE x1.Class = x2.Class 
    ORDER BY x2.Animal 
    FOR XML PATH('')),1,1,'' ) AS "Animals"
FROM @xml x1
GROUP BY Class

See SQL Fiddle with Demo. The result is:

|      CLASS |          ANIMALS |
---------------------------------
| Asteroidea |         Starfish |
|   Mammalia |   Cat,Coyote,Dog |
|   Reptilia | Crocodile,Lizard |