add support UNIQUE

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

@@ -0,0 +1,42 @@
+/*
+ * 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.SQLExpr;
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+@SuppressWarnings("serial")
+public class SQLUnique extends SQLConstaintImpl implements SQLUniqueConstraint, SQLTableElement {
+
+    private final List<SQLExpr> columns = new ArrayList<SQLExpr>();
+
+    public List<SQLExpr> getColumns() {
+        return columns;
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        if (visitor.visit(this)) {
+            acceptChild(visitor, this.getName());
+            acceptChild(visitor, this.getColumns());
+        }
+        visitor.endVisit(this);
+    }
+
+}