SELECT MIN ( drugs ) AS min
     , PERCENTILE_DISC(0.25) WITHIN GROUP( ORDER BY drugs ) 
                                     AS percentile_25
     , ROUND ( AVG ( drugs ), 2 ) AS mean
     , PERCENTILE_DISC(0.5)  WITHIN GROUP (ORDER BY drugs )
                                     AS median
     , PERCENTILE_DISC(0.75) WITHIN GROUP (ORDER BY drugs )
                                     AS percential_75
     , MAX ( drugs ) AS max
     , ROUND ( STDDEV ( drugs ), 1 ) AS stdDev
  FROM -- distinct person/drug
(SELECT person_id, NVL( drugs, 0 ) AS drugs
  FROM person
  LEFT OUTER JOIN
     ( SELECT person_id, COUNT( DISTINCT drug_concept_id ) AS drugs
         FROM drug_exposure
        WHERE drug_exposure_start_date BETWEEN '01-Jan-2008' AND '31-Dec-2008'
        GROUP BY person_id
     ) USING( person_id )
);