map/sample timeseries data to another timeserie db2 - sql

Question

I am trying to combine the results of two SQL (DB2 on IBM bluemix) queries:
The first query creates a timeserie from startdate to enddate:
with dummy(minute) as (
select TIMESTAMP('2017-01-01')
from SYSIBM.SYSDUMMY1 union all
select minute + 1 MINUTES
from dummy
where minute <= TIMESTAMP('2018-01-01')
)
select to_char(minute, 'DD.MM.YYYY HH24:MI') AS minute
from dummy;
The second query selects data from a table which have a timestamp. This data should be joined to the generated timeseries above. The standalone query is like:
SELECT DISTINCT
to_char(date_trunc('minute', TIMESTAMP), 'DD.MM.YYYY HH24:MI') AS minute,
VALUE AS running_ct
FROM TEST
WHERE ID = 'abc'
AND NAME = 'sensor'
ORDER BY minute ASC;
What I suppose to get is a query with one result with contains of two columns:
first column with the timestamp from startdate to enddate and
the second with values which are sorted by there own timestamps to the
first column (empty timestamps=null).
How could I do that?

Answer 1

A better solution, especially if your detail table is large, is to generate a range. This allows the optimizer to use indices to fulfill the bucketing, instead of calling a function on every row (which is expensive).
So something like this:
WITH dummy(temporaer, rangeEnd) AS (SELECT a, a + 1 MINUTE
FROM (VALUES(TIMESTAMP('2017-12-01'))) D(a)
UNION ALL
SELECT rangeEnd, rangeEnd + 1 MINUTE
FROM dummy
WHERE rangeEnd < TIMESTAMP('2018-01-31'))
SELECT Dummy.temporaer, AVG(Test.value) AS TEXT
FROM Dummy
LEFT OUTER JOIN Test
ON Test.timestamp >= Dummy.temporaer
AND Test.timestamp < Dummy.rangeEnd
AND Test.id = 'abc'
AND Test.name = 'text'
GROUP BY Dummy.temporaer
ORDER BY Dummy.temporaer ASC;
Note that the end of the range is now exclusive, not inclusive like you had it before: you were including the very first minute of '2018-01-31', which is probably not what you wanted. Of course, excluding just the last day of a month also strikes me as a little strange - you most likely really want < TIMESTAMP('2018-02-01').

Answer 2

found a working solution:
with dummy(temporaer) as (
select TIMESTAMP('2017-12-01') from SYSIBM.SYSDUMMY1
union all
select temporaer + 1 MINUTES from dummy where temporaer <= TIMESTAMP('2018-01-31'))
select temporaer, avg(VALUE) as text from dummy
LEFT OUTER JOIN TEST ON temporaer=date_trunc('minute', TIMESTAMP) and ID='abc' and NAME='text'
group by temporaer
ORDER BY temporaer ASC;
cheers