UPDATE 
    tree AS main,
    (
        SELECT @rownum:=@rownum+1 AS no, t.id, t.name, t.`depth`, t.parent ,t.seq 
        FROM tree t 
        WHERE (@rownum:=0)=0
        AND t.`type` = 20
        AND t.depth = 2
        AND t.parent = 223
        ORDER BY t.id
    ) AS sub
SET main.seq = sub.`no`
WHERE main.id = sub.id
;