Getting SELECT to return a constant value even if zero rows match

Consider this select statement:

SELECT *, 
       1 AS query_id 
FROM players 
WHERE username='foobar';

It returns the column query_id with value 1 along with a player's other columns.

How would one make the above SQL return at least the query_id of 1 even if the select finds no rows that match?

BTW, it's PostgreSQL 8.4.

SELECT col1, 
       col2, 
       col3, 
       1 AS query_id 
FROM players 
WHERE username='foobar'
union all 
select null,
       null,
       null,
       1
where not exists (select 1 from players where username = 'foobar');

Or as an alternative (might be faster as no second subselect is required):

with qid (query_id) as (
   values (1)
) 
select p.*, 
       qid.query_id
from qid 
  left join players as p on (p.useranme = 'foobar');

You can re-write the above to a more "compact" representation:

select p.*, 
       qid.query_id
from (values (1)) as qid (query_id)
  left join players as p on (p.useranme = 'foobar');

But I think the explicit CTE (with...) is more readable (although that is always in the eyes of the beholder).

StackExchange Database Administrators Q#41067, answer score: 26