>You can't do that, try:
>

>DELETE T1;
>FROM t1 ;
>INNER JOIN (SELECT left(rida,2) AS Rida2,;
>       MAX(month) AS Mnt FROM  t2 ;
>       group by 1) Tbl1;
>ON left(t1.rida,2) = Tbl1.Rida2 AND;
>   t1.Month        = Tbl1.Mnt
>

This deletes MAX(month) rows.
However, I want to delete all other rows so that only MAX(month) rows remain.
There was NOT IN clasue in my original query.

t1.Month # Tbl1.Mnt

condition works