mysql ddl parser support forign key

src/main/java/com/alibaba/druid/sql/ast/statement/SQLForeignKeyConstraint.java

@@ -19,7 +19,7 @@ import java.util.List;
 
 import com.alibaba.druid.sql.ast.SQLName;
 
-public interface SQLForeignKeyConstraint extends SQLConstaint {
+public interface SQLForeignKeyConstraint extends SQLConstaint, SQLTableElement {
 
     List<SQLName> getReferencingColumns();
 

src/main/java/com/alibaba/druid/sql/ast/statement/SQLForeignKeyImpl.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 1999-2011 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.druid.sql.ast.statement;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import com.alibaba.druid.sql.ast.SQLName;
+
+public abstract class SQLForeignKeyImpl extends SQLConstaintImpl implements SQLForeignKeyConstraint {
+
+    private static final long serialVersionUID   = 1L;
+    private SQLName           referencedTableName;
+    private List<SQLName>     referencingColumns = new ArrayList<SQLName>();
+    private List<SQLName>     referencedColumns  = new ArrayList<SQLName>();
+
+    @Override
+    public List<SQLName> getReferencingColumns() {
+        return referencingColumns;
+    }
+
+    @Override
+    public SQLName getReferencedTableName() {
+        return referencedTableName;
+    }
+
+    @Override
+    public void setReferencedTableName(SQLName value) {
+        this.referencedTableName = value;
+    }
+
+    @Override
+    public List<SQLName> getReferencedColumns() {
+        return referencedColumns;
+    }
+
+}