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

There's ways to make this simpler and cleaner, but this basically spells out the technique:

SELECT data_name
FROM data_table
WHERE data_table.type = v_t_id

UNION ALL

SELECT NULL AS data_name
FROM dual
WHERE NOT EXISTS (
    SELECT data_name
    FROM data_table
    WHERE data_table.type = v_t_id
)

When the first part of the union is empty the second will contain a row, when the first part is not empty, the second will contain no rows.

If the query is takes to much time, use this one:

SELECT * FROM (  
    SELECT data_name
    FROM data_table
    WHERE data_table.type = v_t_id

    UNION ALL

    SELECT NULL AS data_name
    FROM dual
  ) WHERE data_name is not null or ROWNUM = 1