dssampdata

SELECT ds.datasetid,

    count(DISTINCT smp.sampleid) AS samples,

    count(DISTINCT dt.dataid) AS observations

   FROM ((ndb.datasets ds

     LEFT JOIN ndb.samples smp ON ((smp.datasetid = ds.datasetid)))

     LEFT JOIN ndb.data dt ON ((dt.sampleid = smp.sampleid)))

  GROUP BY ds.datasetid;