SQL & PL/SQL

Hi

I want to compare one field of x table with another field of y table.
I want rows from x table which does not exists in y.

now i have take comparision like this

where LPAD (x.prod_nbr, 6, '0') || LPAD (x.case_seq_nbr, 2, '0') (+) = TRIM(y.item_id)

bcoz x of two fields making one field in y.

now error is coming for missing expresion. now where should i put outer join??

If i m removing lpad from x.case_seq_nbr then error is gone.

Please help, if anybody can idenfify the problem.

Thanks