SELECT friendID,FU.nickname,FU.image_url,mess_text,date_time
    FROM friends F
        JOIN users U
            ON F.userID=U.userID
        LEFT JOIN users FU
            ON FU.userID=friendID
        JOIN messages M
            ON F.last_message=M.messageID
    WHERE U.userID='$_SESSION[name]'
UNION
SELECT G.groupID,group_name,G.image_url,mess_text,date_time
    FROM groups_users GU
        JOIN users U
            ON GU.userID=U.userID
        LEFT JOIN groups G
            ON GU.groupID=G.groupID
        LEFT JOIN messages M
            ON G.last_message=M.messageID
    WHERE U.userID='$_SESSION[name]'
ORDER BY date_time DESC