I’ve never seen this way of creating a comma delimited list in SQL before and don’t want to loose the logic.

SELECT STUFF
(
    (
        SELECT  ',' + aat.AccessTokenCode
        FROM AssetAccessToken aat
        WHERE aat.AssetID = a.AssetID
        GROUP BY aat.AccessTokenCode
        FOR XML PATH ('')
    ), 1, 1, ''
) as 'AssetAccessTokens'