Re: How to force select to return exactly one row

["Brent Wood" <b.wood@xxxxxxxxxx>]

Use a case staement to test for a null output, & return whatever you want in the event of it being null, else the actual value:

from the top of my head, something like:

SELECT case when
(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') not null

then (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')

else
 0
end

It does require the query to be run twice, so does have extra overhead. You could wrap a function around this to get & store the result & test that, then having stored it you can use it for the output value without a second query. All depends on how much overhead there is in teh query.


HTH,

  Brent Wood

Brent Wood
DBA/GIS consultant
NIWA, Wellington
New Zealand

>>> "Andrus" <kobruleht2@xxxxxx> 06/22/10 10:12 AM >>>

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.


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