Oracle: Get a query to always return exactly one row, even when there's no data to be found

Question

I have a query like this:

   select data_name
   into v_name
   from data_table
   where data_table.type = v_t_id

Normally, this query shou

Answer 1

 select coalesce(data_table.data_name, d.data_name) data_name
   into v_name
   from 
   (SELECT 'UNKNOWN ' as data_name FROM DUAL)  d
   LEFT JOIN data_table
   ON data_table.type = v_t_id
          or a.data_table.data_name is null