An example given below for Oracle Forms, when a value exists then execute query for that value to display the correspondent record else allow user to create a new record for that value.

The following is the example given for HR schema employee table, in this example user will enter an empoyee id and if the employee id exists it will query the record else allow user to create a new record, the trigger written on key-next-item trigger, you can download the employees.fmb form also from the following link employees.fmb

declare
v_empid employees.employee_id%type;
Begin
  Select employee_id into v_empid
     from hr.employees
     where employee_id = :employees.employee_id;
     -- value exists
     -- set block property and execute query
     clear_block(no_validate);
   
     set_block_property('employees', default_where, 'employee_id = '||v_empid);
     execute_query;
     set_block_property('employees', default_where, '');
     next_item;
exception
when no_data_found then
   -- when not then clear block and allow to add new
   v_empid := :employees.employee_id;
   clear_block(no_validate);
   :employees.employee_id := v_empid;
   next_item;
End;