[SPARK-4120][SQL] Join of multiple tables with syntax like SELECT .. FROM...

[SPARK-4120][SQL] Join of multiple tables with syntax like SELECT .. FROM T1,T2,T3.. does not work in SparkSQL

Right now it works for only 2 tables like below query.
sql("SELECT * FROM records1 as a,records2 as b where a.key=b.key ")

But it does not work for more than 2 tables like below query
sql("SELECT * FROM records1 as a,records2 as b,records3 as c where a.key=b.key and a.key=c.key").

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #2987 from ravipesala/multijoin and squashes the following commits:

429b005 [ravipesala] Support multiple joins

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/SqlParser.scala

@@ -166,7 +166,8 @@ class SqlParser extends AbstractSparkSQLParser {
   // Based very loosely on the MySQL Grammar.
   // http://dev.mysql.com/doc/refman/5.0/en/join.html
   protected lazy val relations: Parser[LogicalPlan] =
-    ( relation ~ ("," ~> relation) ^^ { case r1 ~ r2 => Join(r1, r2, Inner, None) }
+    ( relation ~ rep1("," ~> relation) ^^ {
+        case r1 ~ joins => joins.foldLeft(r1) { case(lhs, r) => Join(lhs, r, Inner, None) } }
     | relation
     )
 

sql/core/src/test/scala/org/apache/spark/sql/SQLQuerySuite.scala

@@ -899,4 +899,14 @@ class SQLQuerySuite extends QueryTest with BeforeAndAfterAll {
   test("SPARK-3814 Support Bitwise ~ operator") {
     checkAnswer(sql("SELECT ~key FROM testData WHERE key = 1 "), -2)
   }
+
+  test("SPARK-4120 Join of multiple tables does not work in SparkSQL") {
+    checkAnswer(
+      sql(
+        """SELECT a.key, b.key, c.key
+          |FROM testData a,testData b,testData c
+          |where a.key = b.key and a.key = c.key
+        """.stripMargin),
+      (1 to 100).map(i => Seq(i, i, i)))
+  }
 }