address comments.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/Command.scala

@@ -17,9 +17,14 @@
 
 package org.apache.spark.sql.catalyst.plans.logical
 
+import org.apache.spark.sql.catalyst.expressions.Attribute
+
 /**
  * A logical node that represents a non-query command to be executed by the system.  For example,
  * commands can be used by parsers to represent DDL operations.  Commands, unlike queries, are
  * eagerly executed.
  */
-trait Command
+trait Command extends LeafNode {
+  final override def children: Seq[LogicalPlan] = Seq.empty
+  override def output: Seq[Attribute] = Seq.empty
+}

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/UnsupportedOperationsSuite.scala

@@ -31,10 +31,7 @@ import org.apache.spark.sql.streaming.OutputMode
 import org.apache.spark.sql.types.IntegerType
 
 /** A dummy command for testing unsupported operations. */
-case class DummyCommand() extends LogicalPlan with Command {
-  override def output: Seq[Attribute] = Nil
-  override def children: Seq[LogicalPlan] = Nil
-}
+case class DummyCommand() extends Command
 
 class UnsupportedOperationsSuite extends SparkFunSuite {
 

sql/core/src/main/scala/org/apache/spark/sql/execution/command/SetCommand.scala

@@ -129,6 +129,4 @@ case object ResetCommand extends RunnableCommand with Logging {
     sparkSession.sessionState.conf.clear()
     Seq.empty[Row]
   }
-
-  override val output: Seq[Attribute] = Seq.empty
 }

sql/core/src/main/scala/org/apache/spark/sql/execution/command/cache.scala

@@ -19,7 +19,6 @@ package org.apache.spark.sql.execution.command
 
 import org.apache.spark.sql.{Dataset, Row, SparkSession}
 import org.apache.spark.sql.catalyst.TableIdentifier
-import org.apache.spark.sql.catalyst.expressions.Attribute
 import org.apache.spark.sql.catalyst.plans.QueryPlan
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 
@@ -47,8 +46,6 @@ case class CacheTableCommand(
 
     Seq.empty[Row]
   }
-
-  override def output: Seq[Attribute] = Seq.empty
 }
 
 
@@ -58,8 +55,6 @@ case class UncacheTableCommand(tableIdent: TableIdentifier) extends RunnableComm
     sparkSession.catalog.uncacheTable(tableIdent.quotedString)
     Seq.empty[Row]
   }
-
-  override def output: Seq[Attribute] = Seq.empty
 }
 
 /**
@@ -71,6 +66,4 @@ case object ClearCacheCommand extends RunnableCommand {
     sparkSession.catalog.clearCache()
     Seq.empty[Row]
   }
-
-  override def output: Seq[Attribute] = Seq.empty
 }

sql/core/src/main/scala/org/apache/spark/sql/execution/command/commands.scala

@@ -35,9 +35,7 @@ import org.apache.spark.sql.types._
  * A logical command that is executed for its side-effects.  `RunnableCommand`s are
  * wrapped in `ExecutedCommand` during execution.
  */
-trait RunnableCommand extends LogicalPlan with logical.Command {
-  override def output: Seq[Attribute] = Seq.empty
-  final override def children: Seq[LogicalPlan] = Seq.empty
+trait RunnableCommand extends logical.Command {
   def run(sparkSession: SparkSession): Seq[Row]
 }
 

sql/core/src/main/scala/org/apache/spark/sql/execution/command/databases.scala

@@ -59,6 +59,4 @@ case class SetDatabaseCommand(databaseName: String) extends RunnableCommand {
     sparkSession.sessionState.catalog.setCurrentDatabase(databaseName)
     Seq.empty[Row]
   }
-
-  override val output: Seq[Attribute] = Seq.empty
 }

sql/core/src/main/scala/org/apache/spark/sql/execution/command/ddl.scala

@@ -70,8 +70,6 @@ case class CreateDatabaseCommand(
       ifNotExists)
     Seq.empty[Row]
   }
-
-  override val output: Seq[Attribute] = Seq.empty
 }
 
 
@@ -101,8 +99,6 @@ case class DropDatabaseCommand(
     sparkSession.sessionState.catalog.dropDatabase(databaseName, ifExists, cascade)
     Seq.empty[Row]
   }
-
-  override val output: Seq[Attribute] = Seq.empty
 }
 
 /**
@@ -126,8 +122,6 @@ case class AlterDatabasePropertiesCommand(
 
     Seq.empty[Row]
   }
-
-  override val output: Seq[Attribute] = Seq.empty
 }
 
 /**

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

@@ -20,14 +20,15 @@ package org.apache.spark.sql.execution.datasources
 import org.apache.spark.sql._
 import org.apache.spark.sql.catalyst.TableIdentifier
 import org.apache.spark.sql.catalyst.catalog.CatalogTable
-import org.apache.spark.sql.catalyst.expressions.Attribute
 import org.apache.spark.sql.catalyst.plans.QueryPlan
 import org.apache.spark.sql.catalyst.plans.logical.{Command, LogicalPlan}
 import org.apache.spark.sql.execution.command.RunnableCommand
 import org.apache.spark.sql.types._
 
-case class CreateTable(tableDesc: CatalogTable, mode: SaveMode, query: Option[LogicalPlan])
-  extends LogicalPlan with Command {
+case class CreateTable(
+    tableDesc: CatalogTable,
+    mode: SaveMode,
+    query: Option[LogicalPlan]) extends Command {
   assert(tableDesc.provider.isDefined, "The table to be created must have a provider.")
 
   if (query.isEmpty) {
@@ -36,10 +37,6 @@ case class CreateTable(tableDesc: CatalogTable, mode: SaveMode, query: Option[Lo
       "create table without data insertion can only use ErrorIfExists or Ignore as SaveMode.")
   }
 
-  override def output: Seq[Attribute] = Seq.empty[Attribute]
-
-  override def children: Seq[LogicalPlan] = Seq.empty[LogicalPlan]
-
   override def innerChildren: Seq[QueryPlan[_]] = query.toSeq
 }