How to include US states with zero counts() in Oracle SQL? - sql

Question

I'm struggling to craft a query for an Oracle db that will return "0" for each US state that is not represented in the results of this query:
SELECT TBL_STATES.STATE_ABBR as State, count(tbl_stations.station_state) AS Observations
FROM TBL_STATES
LEFT JOIN TBL_STATIONS ON TBL_STATES.STATE_ABBR = TBL_STATIONS.STATION_STATE
LEFT JOIN TBL_OBSERVATIONS ON TBL_STATIONS.STATION_ID = TBL_OBSERVATIONS.STATION_ID
WHERE EXTRACT(year FROM TBL_OBSERVATIONS.OBSERVATION_DATE)='2015'
AND EXTRACT(month FROM TBL_OBSERVATIONS.OBSERVATION_DATE)='8'
GROUP BY STATE_ABBR
ORDER BY STATE_ABBR
The query currently returns a count for each state in which an observation has been made, as shown here:
STATE | OBSERVATIONS
AZ | 131
CA | 30
CO | 9
FL | 6
...and so on.
What I'd like to see is a count for every entry in TBL_STATES (which contains records for all 50 states + DC & PR):
STATE | OBSERVATIONS
AK | 0
AL | 0
AR | 0
AZ | 131
CA | 30
CO | 9
CT | 0
DC | 0
DE | 0
FL | 6
...etc.
I've also attempted variations of NVL(count(TBL_STATES.STATE_ABBR),0) without success.
What the heck am I missing here?

Answer 1

Just move the date conditions to the ON clause:
SELECT TBL_STATES.STATE_ABBR as State, count(o.station_id) AS Observations
FROM TBL_STATES LEFT JOIN
TBL_STATIONS
ON TBL_STATES.STATE_ABBR = TBL_STATIONS.STATION_STATE LEFT JOIN
TBL_OBSERVATIONS
ON TBL_STATIONS.STATION_ID = TBL_OBSERVATIONS.STATION_ID AND
EXTRACT(year FROM TBL_OBSERVATIONS.OBSERVATION_DATE) = 2015
EXTRACT(month FROM TBL_OBSERVATIONS.OBSERVATION_DATE) = 8
GROUP BY STATE_ABBR
ORDER BY STATE_ABBR;
Because you seem to want to count observations, I also fixed the COUNT() to be from the observations table. This is important when the left join does not find a match.
I am a big fan of using table aliases. Also, direct date comparisons can be more efficient than extract(). So, I would recommend:
SELECT st.STATE_ABBR as State, count(o.STATION_ID) AS Observations
FROM TBL_STATES st LEFT JOIN
TBL_STATIONS sta
ON st.STATE_ABBR = sta.STATION_STATE LEFT JOIN
TBL_OBSERVATIONS o
ON sta.STATION_ID = o.STATION_ID AND
o.OBSERVATION_DATE >= DATE '2015-08-01' AND
o.OBSERVATION_DATE < DATE '2015-09-01'
GROUP BY st.STATE_ABBR
ORDER BY st.STATE_ABBR;

Answer 2

Change the WHERE clause to AND moving the criteria to the join. The where clause is negating the left join making it an inner.
we may also have to use a sub query to limit the results first then join... or perhaps you need to be counting the occurrences of observation_date instead of station_state, since year isn't in tbl_stations...
SELECT TBL_STATES.STATE_ABBR as State, count(b.ovservation_date) AS Observations
FROM TBL_STATES
LEFT JOIN TBL_STATIONS ON TBL_STATES.STATE_ABBR = TBL_STATIONS.STATION_STATE
LEFT JOIN (SELECT * FROM TBL_OBSERVATIONS
WHERE EXTRACT(year FROM TBL_OBSERVATIONS.OBSERVATION_DATE)='2015'
AND EXTRACT(month FROM TBL_OBSERVATIONS.OBSERVATION_DATE)='8') B
ON TBL_STATIONS.STATION_ID = B.STATION_ID
GROUP BY STATE_ABBR
ORDER BY STATE_ABBR
Logically the left joins are returning NULL counts for those states... but then the where clause is ELIMINATING the records because there is no observation date thus NULL <> 2015, and the records are being excluded, negating the left join.