How to set a default row for a query that returns no rows?

Question

I need to know how to return a default row if no rows exist in a table. What would be the best way to do this? I\'m only returning a single column from this particular table

Answer 1

Assuming there is a table config with unique index on config_code column:

CONFIG_CODE     PARAM1   PARAM2
--------------- -------- --------
default_config  def      000
config1         abc      123
config2         def      456

This query returns line for config1 values, because it exists in the table:

SELECT *
  FROM (SELECT *
          FROM config
         WHERE config_code = 'config1'
            OR config_code = 'default_config'
         ORDER BY CASE config_code WHEN 'default_config' THEN 999 ELSE 1 END)
 WHERE rownum = 1;

CONFIG_CODE     PARAM1   PARAM2
--------------- -------- --------
config1         abc      123

This one returns default record as config3 doesn't exist in the table:

SELECT *
  FROM (SELECT *
          FROM config
         WHERE config_code = 'config3'
            OR config_code = 'default_config'
         ORDER BY CASE config_code WHEN 'default_config' THEN 999 ELSE 1 END)
 WHERE rownum = 1;

CONFIG_CODE     PARAM1   PARAM2
--------------- -------- --------
default_config  def      000

In comparison with other solutions this one queries table config only once.