INSERT OVERWRITE TABLE output_table
SELECT  a,b,c
         from_unixtime(unix_timestamp(snapshot_date_time, 'yyyy-MM-dd')) AS snapshot_date_time ,
         count (DISTINCT a)  AS table_c , (
         CASE
                  WHEN (
                                    c>=0 ) THEN count (DISTINCT a)
                  ELSE 0
         END)                     AS dist_a ,
         count (DISTINCT b) AS dist_b ,
FROM     (
                SELECT a,b,c
                       date_sub(to_date(from_unixtime(unix_timestamp())),117) AS snapshot_date_time
                FROM   table_a
                WHERE  to_date(start_date_time)<= date_sub(to_date(from_unixtime(unix_timestamp())),117)
                AND    (
                              to_date(end_date_time)> date_sub(to_date(from_unixtime(unix_timestamp())),117)
                       OR     end_date_time IS NULL)
                UNION ALL
                SELECT a,b,c
                       date_sub(to_date(from_unixtime(unix_timestamp())),118) AS snapshot_date_time ,
                FROM   table_b
                WHERE  to_date(start_date_time)<= date_sub(to_date(from_unixtime(unix_timestamp())),118)
                AND    (
                              to_date(end_date_time)> date_sub(to_date(from_unixtime(unix_timestamp())),118)
                       OR     end_date_time IS NULL)
               ) table_b
GROUP BY a,b,c;