sql group by max

SELECT
    * ,
    REPLACE(TDFG.xdfd,'doc_111','') GB
FROM
    (
        SELECT
            *
        FROM
            (
                SELECT
                    *
                FROM
                    (
                        SELECT
                            *,
                            CASE sender
                                WHEN 'doc_111'
                                THEN CONCAT(receiver,sender)
                                ELSE CONCAT(sender,receiver)
                            END 'xdfd'
                        FROM
                            user_msg )TTTT
                ORDER BY
                    create_time DESC) t
        GROUP BY
            xdfd ) TDFG
INNER JOIN
    user_doc SSD
ON
    SSD.id=REPLACE(TDFG.xdfd,'doc_111','')