How to force select to return exactly one row

Autogenerated select statement contains 0 .. n left joins:

SELECT somecolumns
FROM ko
LEFT JOIN t1 ON t1.primarykey= ko.t1foreignkwey
...
LEFT JOIN tn ON tn.primarykey= ko.tnforeignkwey
WHERE ko.primarykey='someprimarykeyvalue';

This select can return only 0 or 1 rows depending if ko row with primary key
'someprimarykeyvalue' exists or not.

Problem:

if there is no searched primary key row in ko database, select should also
return empty row.

To get this result I added right join:

SELECT somecolumns
FROM ko
RIGHT JOIN (SELECT 1) _forceonerow ON true
LEFT JOIN t1 ON t1.primarykey= ko.t1foreignkwey
...
LEFT JOIN tn ON tn.primarykey= ko.tnforeignkwey
WHERE ko.primarykey is null or ko.primarykey='someprimarykeyvalue';

but it still does not return row if primary key row 'someprimarykeyvalue'
does not exist.

How to force this statement to return one row always ?

Andrus.