[SPARK-25736][SQL][TEST] add tests to verify the behavior of multi-column count

## What changes were proposed in this pull request?

AFAIK multi-column count is not widely supported by the mainstream databases(postgres doesn't support), and the SQL standard doesn't define it clearly, as near as I can tell.

Since Spark supports it, we should clearly document the current behavior and add tests to verify it.

## How was this patch tested?

N/A

Closes #22728 from cloud-fan/doc.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: hyukjinkwon <gurwls223@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Count.scala

@@ -52,7 +52,7 @@ abstract class CountLike extends DeclarativeAggregate {
   usage = """
     _FUNC_(*) - Returns the total number of retrieved rows, including rows containing null.
 
-    _FUNC_(expr) - Returns the number of rows for which the supplied expression is non-null.
+    _FUNC_(expr[, expr...]) - Returns the number of rows for which the supplied expression(s) are all non-null.
 
     _FUNC_(DISTINCT expr[, expr...]) - Returns the number of rows for which the supplied expression(s) are unique and non-null.
   """)

sql/core/src/test/resources/sql-tests/inputs/count.sql

@@ -0,0 +1,27 @@
+-- Test data.
+CREATE OR REPLACE TEMPORARY VIEW testData AS SELECT * FROM VALUES
+(1, 1), (1, 2), (2, 1), (1, 1), (null, 2), (1, null), (null, null)
+AS testData(a, b);
+
+-- count with single expression
+SELECT
+  count(*), count(1), count(null), count(a), count(b), count(a + b), count((a, b))
+FROM testData;
+
+-- distinct count with single expression
+SELECT
+  count(DISTINCT 1),
+  count(DISTINCT null),
+  count(DISTINCT a),
+  count(DISTINCT b),
+  count(DISTINCT (a + b)),
+  count(DISTINCT (a, b))
+FROM testData;
+
+-- count with multiple expressions
+SELECT count(a, b), count(b, a), count(testData.*) FROM testData;
+
+-- distinct count with multiple expressions
+SELECT
+  count(DISTINCT a, b), count(DISTINCT b, a), count(DISTINCT *), count(DISTINCT testData.*)
+FROM testData;

sql/core/src/test/resources/sql-tests/results/count.sql.out

@@ -0,0 +1,55 @@
+-- Automatically generated by SQLQueryTestSuite
+-- Number of queries: 5
+
+
+-- !query 0
+CREATE OR REPLACE TEMPORARY VIEW testData AS SELECT * FROM VALUES
+(1, 1), (1, 2), (2, 1), (1, 1), (null, 2), (1, null), (null, null)
+AS testData(a, b)
+-- !query 0 schema
+struct<>
+-- !query 0 output
+
+
+
+-- !query 1
+SELECT
+  count(*), count(1), count(null), count(a), count(b), count(a + b), count((a, b))
+FROM testData
+-- !query 1 schema
+struct<count(1):bigint,count(1):bigint,count(NULL):bigint,count(a):bigint,count(b):bigint,count((a + b)):bigint,count(named_struct(a, a, b, b)):bigint>
+-- !query 1 output
+7	7	0	5	5	4	7
+
+
+-- !query 2
+SELECT
+  count(DISTINCT 1),
+  count(DISTINCT null),
+  count(DISTINCT a),
+  count(DISTINCT b),
+  count(DISTINCT (a + b)),
+  count(DISTINCT (a, b))
+FROM testData
+-- !query 2 schema
+struct<count(DISTINCT 1):bigint,count(DISTINCT NULL):bigint,count(DISTINCT a):bigint,count(DISTINCT b):bigint,count(DISTINCT (a + b)):bigint,count(DISTINCT named_struct(a, a, b, b)):bigint>
+-- !query 2 output
+1	0	2	2	2	6
+
+
+-- !query 3
+SELECT count(a, b), count(b, a), count(testData.*) FROM testData
+-- !query 3 schema
+struct<count(a, b):bigint,count(b, a):bigint,count(a, b):bigint>
+-- !query 3 output
+4	4	4
+
+
+-- !query 4
+SELECT
+  count(DISTINCT a, b), count(DISTINCT b, a), count(DISTINCT *), count(DISTINCT testData.*)
+FROM testData
+-- !query 4 schema
+struct<count(DISTINCT a, b):bigint,count(DISTINCT b, a):bigint,count(DISTINCT a, b):bigint,count(DISTINCT a, b):bigint>
+-- !query 4 output
+3	3	3	3