I have a DATE column that I want to round to the next-lower 10 minute interval in a query (see example below).

I managed to do it by truncating the seconds and then subtracting the last digit of minutes.

WITH test_data AS (
        SELECT TO_DATE('2010-01-01 10:00:00', 'YYYY-MM-DD HH24:MI:SS') d FROM dual
  UNION SELECT TO_DATE('2010-01-01 10:05:00', 'YYYY-MM-DD HH24:MI:SS') d FROM dual
  UNION SELECT TO_DATE('2010-01-01 10:09:59', 'YYYY-MM-DD HH24:MI:SS') d FROM dual
  UNION SELECT TO_DATE('2010-01-01 10:10:00', 'YYYY-MM-DD HH24:MI:SS') d FROM dual
  UNION SELECT TO_DATE('2099-01-01 10:00:33', 'YYYY-MM-DD HH24:MI:SS') d FROM dual
)
-- #end of test-data
SELECT
  d, TRUNC(d, 'MI') - MOD(TO_CHAR(d, 'MI'), 10) / (24 * 60)
FROM test_data

And here is the result:

01.01.2010 10:00:00    01.01.2010 10:00:00
01.01.2010 10:05:00    01.01.2010 10:00:00
01.01.2010 10:09:59    01.01.2010 10:00:00
01.01.2010 10:10:00    01.01.2010 10:10:00
01.01.2099 10:00:33    01.01.2099 10:00:00

Works as expected, but is there a better way?

EDIT:

I was curious about performance, so I did the following test with 500.000 rows and (not really) random dates. I am going to add the results as comments to the provided solutions.

DECLARE
  t       TIMESTAMP := SYSTIMESTAMP;
BEGIN
  FOR i IN (
    WITH test_data AS (
      SELECT SYSDATE + ROWNUM / 5000 d FROM dual
      CONNECT BY ROWNUM <= 500000
    )
    SELECT TRUNC(d, 'MI') - MOD(TO_CHAR(d, 'MI'), 10) / (24 * 60)
    FROM test_data
  )
  LOOP
    NULL;
  END LOOP;
  dbms_output.put_line( SYSTIMESTAMP - t );
END;

This approach took 03.24 s.