[SPARK-14039][SQL][MINOR] make SubqueryHolder an inner class

## What changes were proposed in this pull request?

`SubqueryHolder` is only used when generate SQL string in `SQLBuilder`, it's more clear to make it an inner class in `SQLBuilder`.

## How was this patch tested?

existing tests.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11861 from cloud-fan/gensql.

1 files changed, 9 insertions, 9 deletions

diff --git a/sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala b/sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala
index 5175bd4609..b2196c4409 100644
--- a/sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala
+++ b/sql/hive/src/main/scala/org/apache/spark/sql/hive/SQLBuilder.scala
@@ -34,15 +34,6 @@ import org.apache.spark.sql.hive.execution.HiveScriptIOSchema
 import org.apache.spark.sql.types.{ByteType, DataType, IntegerType, NullType}
 
 /**
- * A place holder for generated SQL for subquery expression.
- */
-case class SubqueryHolder(query: String) extends LeafExpression with Unevaluable {
-  override def dataType: DataType = NullType
-  override def nullable: Boolean = true
-  override def sql: String = s"($query)"
-}
-
-/**
  * A builder class used to convert a resolved logical plan into a SQL query string.  Note that not
  * all resolved logical plan are convertible.  They either don't have corresponding SQL
  * representations (e.g. logical plans that operate on local Scala collections), or are simply not
@@ -530,4 +521,13 @@ class SQLBuilder(logicalPlan: LogicalPlan, sqlContext: SQLContext) extends Loggi
       case _ => None
     }
   }
+
+  /**
+   * A place holder for generated SQL for subquery expression.
+   */
+  case class SubqueryHolder(query: String) extends LeafExpression with Unevaluable {
+    override def dataType: DataType = NullType
+    override def nullable: Boolean = true
+    override def sql: String = s"($query)"
+  }
 }