[SPARK-8621] [SQL] support empty string as column name

improve the empty check in `parseAttributeName` so that we can allow empty string as column name.
Close https://github.com/apache/spark/pull/7117

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7149 from cloud-fan/8621 and squashes the following commits:

efa9e3e [Wenchen Fan] support empty string

(cherry picked from commit 31b4a3d7f2be9053a041e5ae67418562a93d80d8)
Signed-off-by: Reynold Xin <rxin@databricks.com>

2 files changed, 9 insertions, 2 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala
index dba69659af..5567189c42 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala
@@ -163,7 +163,7 @@ abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
           if (tmp.nonEmpty) throw e
           inBacktick = true
         } else if (char == '.') {
-          if (tmp.isEmpty) throw e
+          if (name(i - 1) == '.' || i == name.length - 1) throw e
           nameParts += tmp.mkString
           tmp.clear()
         } else {
@@ -172,7 +172,7 @@ abstract class LogicalPlan extends QueryPlan[LogicalPlan] with Logging {
       }
       i += 1
     }
-    if (tmp.isEmpty || inBacktick) throw e
+    if (inBacktick) throw e
     nameParts += tmp.mkString
     nameParts.toSeq
   }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
index 8e71ef9683..399ab2bb2e 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/DataFrameSuite.scala
@@ -638,4 +638,11 @@ class DataFrameSuite extends QueryTest {
     val res11 = TestSQLContext.range(-1).select("id")
     assert(res11.count == 0)
   }
+
+  test("SPARK-8621: support empty string column name") {
+    val df = Seq(Tuple1(1)).toDF("").as("t")
+    // We should allow empty string as column name
+    df.col("")
+    df.col("t.``")
+  }
 }