diff --git a/src/Sql/AbstractSql.php b/src/Sql/AbstractSql.php
index 30f83379..7e89c8c4 100644
--- a/src/Sql/AbstractSql.php
+++ b/src/Sql/AbstractSql.php
@@ -35,8 +35,8 @@
 use function vsprintf;
 
 abstract class AbstractSql implements SqlInterface
-{
-    public ?object $subject = null;
+    {
+    protected SqlInterface|PreparableSqlInterface|null $subject = null;
 
     /**
      * Specifications for Sql String generation
diff --git a/src/Sql/Ddl/AlterTable.php b/src/Sql/Ddl/AlterTable.php
index 29361cbb..3d7be7cc 100644
--- a/src/Sql/Ddl/AlterTable.php
+++ b/src/Sql/Ddl/AlterTable.php
@@ -10,7 +10,7 @@
 
 use function array_key_exists;
 
-class AlterTable extends AbstractSql implements SqlInterface
+class AlterTable extends AbstractSql
 {
     final public const ADD_COLUMNS = 'addColumns';
 
diff --git a/src/Sql/Ddl/CreateTable.php b/src/Sql/Ddl/CreateTable.php
index fd4e1ac1..1ccfa494 100644
--- a/src/Sql/Ddl/CreateTable.php
+++ b/src/Sql/Ddl/CreateTable.php
@@ -10,7 +10,7 @@
 
 use function array_key_exists;
 
-class CreateTable extends AbstractSql implements SqlInterface
+class CreateTable extends AbstractSql
 {
     final public const COLUMNS = 'columns';
 
diff --git a/src/Sql/Ddl/DropTable.php b/src/Sql/Ddl/DropTable.php
index 2423c604..6193f939 100644
--- a/src/Sql/Ddl/DropTable.php
+++ b/src/Sql/Ddl/DropTable.php
@@ -8,7 +8,7 @@
 use PhpDb\Sql\AbstractSql;
 use PhpDb\Sql\TableIdentifier;
 
-class DropTable extends AbstractSql implements SqlInterface
+class DropTable extends AbstractSql
 {
     final public const TABLE = 'table';
 
diff --git a/src/Sql/Ddl/SqlInterface.php b/src/Sql/Ddl/SqlInterface.php
deleted file mode 100644
index 022f6c1e..00000000
--- a/src/Sql/Ddl/SqlInterface.php
+++ /dev/null
@@ -1,11 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace PhpDb\Sql\Ddl;
-
-use PhpDb\Sql\SqlInterface as BaseSqlInterface;
-
-interface SqlInterface extends BaseSqlInterface
-{
-}
diff --git a/src/Sql/Platform/PlatformDecoratorInterface.php b/src/Sql/Platform/PlatformDecoratorInterface.php
index f35528eb..b39bedba 100644
--- a/src/Sql/Platform/PlatformDecoratorInterface.php
+++ b/src/Sql/Platform/PlatformDecoratorInterface.php
@@ -4,7 +4,12 @@
 
 namespace PhpDb\Sql\Platform;
 
+use PhpDb\Sql\PreparableSqlInterface;
+use PhpDb\Sql\SqlInterface;
+
 interface PlatformDecoratorInterface
 {
-    public function setSubject(?object $subject): PlatformDecoratorInterface;
+    public function setSubject(
+        SqlInterface|PreparableSqlInterface|null $subject
+    ): PlatformDecoratorInterface;
 }