SQL Server主从表字段拼接更新

UPDATE main
SET main.SystemDocNoList = left(sub.CombinedSystemDocNo,999)
FROM tb_DO main
INNER JOIN (
    SELECT 
        DONO,
        -- 去掉最前面多余的逗号
        STUFF(
            (SELECT ',' + detail.SystemDocNo
             FROM tb_DOs detail
             WHERE detail.DONO = main.DONO
               AND detail.SystemDocNo IS NOT NULL 
               AND detail.SystemDocNo <> ''
             FOR XML PATH(''), TYPE).value('.', 'NVARCHAR(MAX)'),
        1, 1, '') AS CombinedSystemDocNo
    FROM tb_DOs main 
	where isnull(SystemDocNo,'')!='' --过滤空数据
    GROUP BY DONO
) sub 
ON main.DONO = sub.DONO
where isnull(main.SystemDocNoList,'') != left(isnull(sub.CombinedSystemDocNo,''),999);