postgres function - declare variable with select statement - postgresql

Question

i have modifed my function, but i have problems with declare variables. I used postgres 8.4. Have someone an idea?
Function:
CREATE OR REPLACE FUNCTION requestcounterid(_mindate timestamptz, _maxdate timestamptz)
RETURNS TABLE (kategorien text, requestcounter int) AS
$func$
DECLARE
_minid bigint;
_maxid bigint;
BEGIN
_minid := (SELECT id from tablename where starttime >= $1 ORDER BY tablename2 ASC LIMIT 1);
_maxid := (SELECT id from tablename where starttime < $2 ORDER BY tablename2 DESC LIMIT 1);
SELECT CASE WHEN duration <= 10000000 THEN '00-01 sec'::text
WHEN duration <= 40000000 THEN '01-04 sec'
WHEN duration <= 100000000 THEN '04-10 sec'
WHEN duration <= 300000000 THEN '10-30 sec'
WHEN duration <= 600000000 THEN '30-60 sec'
ELSE 'more than 60 sec' END
, count(*)::int
FROM tablename
WHERE id >= _minid and id <= _maxid
GROUP BY 1
ORDER BY 1;
END;
$func$ LANGUAGE plpgsql;
Error:
ERROR: query has no destination for result data
HINT: If you want to discard the results of a SELECT, use PERFORM instead.
CONTEXT: PL/pgSQL function "requestcounterid" line 11 at SQL statement
Regrads

Answer 1

Now it workes:
CREATE OR REPLACE FUNCTION requestcounterid(_mindate timestamptz, _maxdate timestamptz)
RETURNS TABLE (kategorien text, requestcounter int) AS
$func$
DECLARE
_minid bigint;
_maxid bigint;
BEGIN
SELECT id INTO _minid from tablename where starttime >= $1 ORDER BY starttime ASC LIMIT 1;
SELECT id INTO _maxid from tablename where starttime < $2 ORDER BY starttime DESC LIMIT 1;
Return Query SELECT CASE WHEN duration <= 10000000 THEN '00-01 sec'::text
WHEN duration <= 40000000 THEN '01-04 sec'
WHEN duration <= 100000000 THEN '04-10 sec'
WHEN duration <= 300000000 THEN '10-30 sec'
WHEN duration <= 600000000 THEN '30-60 sec'
ELSE 'more than 60 sec' END
, count(*)::int
FROM tablename
WHERE id >= _minid and id <= _maxid
GROUP BY 1
ORDER BY 1;
END;
$func$ LANGUAGE plpgsql;