Add comparison IT and doctest

docs/user/dql/aggregations.rst

@@ -126,6 +126,14 @@ The aggregation could has expression as arguments::
     | M        | 202    |
     +----------+--------+
 
+COUNT Aggregations
+------------------
+
+Besides regular identifiers, ``COUNT`` aggregate function also accepts arguments such as ``*`` or literals like ``1``. The meaning of these different forms are as follows:
+
+1. ``COUNT(field)`` will count only if given field (or expression) is not null or missing in the input rows.
+2. ``COUNT(*)`` will count the number of all its input rows.
+3. ``COUNT(1)`` is same as ``COUNT(*)`` because any non-null literal will count.
 
 HAVING Clause
 =============

integ-test/src/test/resources/correctness/queries/groupby.txt

@@ -1,8 +1,16 @@
 SELECT COUNT(*) FROM kibana_sample_data_flights
+SELECT COUNT(1) FROM kibana_sample_data_flights
+SELECT COUNT('hello') FROM kibana_sample_data_flights
 SELECT SUM(FlightDelayMin) FROM kibana_sample_data_flights
 SELECT AVG(FlightDelayMin) FROM kibana_sample_data_flights
+SELECT MIN(FlightDelayMin) FROM kibana_sample_data_flights
+SELECT MAX(FlightDelayMin) FROM kibana_sample_data_flights
 SELECT count(*), Avg(FlightDelayMin), sUm(FlightDelayMin) FROM kibana_sample_data_flights
 SELECT COUNT(*) AS cnt, AVG(FlightDelayMin) AS a, SUM(FlightDelayMin) AS s FROM kibana_sample_data_flights
+SELECT COUNT(*) FROM kibana_sample_data_flights WHERE FlightTimeMin > 0
+SELECT COUNT(1) FROM kibana_sample_data_flights WHERE FlightTimeMin > 0
+SELECT SUM(FlightDelayMin) FROM kibana_sample_data_flights WHERE FlightTimeMin > 0
+SELECT AVG(FlightDelayMin) FROM kibana_sample_data_flights WHERE FlightTimeMin > 0
 SELECT COUNT(*) FROM kibana_sample_data_flights GROUP BY OriginCountry
 SELECT COUNT(FlightNum) FROM kibana_sample_data_flights GROUP BY OriginCountry
 SELECT COUNT(FlightDelay) FROM kibana_sample_data_flights GROUP BY OriginCountry