This does not work.

SELECT left(rida,2) + MAX(month) FROM t2 ;
group by 1

causes error

Cannot GROUP by aggregate field.



>You cannot compare multiple columns that way. Instead concatenate the rida and month columns together and compare them in your WHERE statement such as:
>

>DELETE FROM t1 ;
>  WHERE ( left(rida,2) + month ) NOT IN ;
>   (SELECT left(rida,2) +  MAX(month) FROM  t2 ;
>   group by 1)
>