content: /*
   * 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.support.opds.udf;
  
  import com.alibaba.druid.DbType;
  import com.alibaba.druid.sql.SQLUtils;
  import com.alibaba.druid.sql.ast.SQLStatement;
  import com.alibaba.druid.sql.visitor.SchemaStatVisitor;
  import com.alibaba.druid.stat.TableStat;
  import com.aliyun.odps.udf.UDF;
  
  import java.util.List;
  
  public class ExportColumns extends UDF {
      public String evaluate(String sql) {
          return evaluate(sql, null);
      }
  
      public String evaluate(String sql, String dbType) {
          try {
              List<SQLStatement> statementList = SQLUtils.parseStatements(sql, DbType.valueOf(dbType));
              SchemaStatVisitor visitor = SQLUtils.createSchemaStatVisitor(DbType.valueOf(dbType));
  
              for (SQLStatement stmt : statementList) {
                  stmt.accept(visitor);
              }
  
              StringBuilder buf = new StringBuilder();
  
              for (TableStat.Column column : visitor.getColumns()) {
                  if (buf.length() != 0) {
                      buf.append(',');
                  }
                  buf.append(column.toString());
              }
  
              return buf.toString();
          } catch (Throwable ex) {
              return null;
          }
      }
  }
  