oracle - enable an end user to type in - pl/sql -

i need write plsql program enable end user type in customer_id. display list of unique product_ids customer purchased sales table.

begin   p in (select unique product_id sales)   loop     dbms_output.put_line( 'customer id: ' || p.product_id );   end loop; end; 

i have solved second part of displaying unique values follows not sure how enable end user type in customer_id?? help

you can use ampersend (&) sign placeholder in pl/sql block. :

begin   p in (select unique product_id sales customer_id = &1)   loop      dbms_output.put_line( 'customer id: ' || p.product_id );   end loop; end;