Add a case for isnotnull operator

Author: HyukjinKwon

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -285,6 +285,7 @@ private[sql] class JDBCRDD(
     case GreaterThan(attr, value) => s"$attr > ${compileValue(value)}"
     case LessThanOrEqual(attr, value) => s"$attr <= ${compileValue(value)}"
     case GreaterThanOrEqual(attr, value) => s"$attr >= ${compileValue(value)}"
+    case IsNotNull(attr) => s"$attr IS NOT NULL"
     case _ => null
   }
 

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -182,6 +182,7 @@ class JDBCSuite extends SparkFunSuite with BeforeAndAfter with SharedSQLContext
     assert(sql("SELECT * FROM foobar WHERE NAME = 'fred'").collect().size === 1)
     assert(sql("SELECT * FROM foobar WHERE NAME > 'fred'").collect().size === 2)
     assert(sql("SELECT * FROM foobar WHERE NAME != 'fred'").collect().size === 2)
+    assert(sql("SELECT * FROM nulltypes WHERE A IS NOT NULL").collect().size === 0)
   }
 
   test("SELECT * WHERE (quoted strings)") {