refactor sql parser.

src/main/java/com/alibaba/druid/sql/ast/SQLArgument.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 1999-2017 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;
 
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;

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

@@ -15,6 +15,7 @@
  */
 package com.alibaba.druid.sql.ast.statement;
 
+import com.alibaba.druid.sql.ast.SQLLimit;
 import com.alibaba.druid.sql.ast.SQLObjectImpl;
 import com.alibaba.druid.sql.ast.SQLOrderBy;
 import com.alibaba.druid.sql.visitor.SQLASTVisitor;
@@ -26,6 +27,8 @@ public class SQLUnionQuery extends SQLObjectImpl implements SQLSelectQuery {
     private SQLUnionOperator operator = SQLUnionOperator.UNION;
     private SQLOrderBy       orderBy;
 
+    private SQLLimit limit;
+
     public SQLUnionOperator getOperator() {
         return operator;
     }
@@ -77,8 +80,20 @@ public class SQLUnionQuery extends SQLObjectImpl implements SQLSelectQuery {
             acceptChild(visitor, left);
             acceptChild(visitor, right);
             acceptChild(visitor, orderBy);
+            acceptChild(visitor, limit);
         }
         visitor.endVisit(this);
     }
 
+
+    public SQLLimit getLimit() {
+        return limit;
+    }
+
+    public void setLimit(SQLLimit limit) {
+        if (limit != null) {
+            limit.setParent(this);
+        }
+        this.limit = limit;
+    }
 }