如何在Java中两次使用同一结果集

Question

我想两次使用相同的结果集，一次是检查它是否为空并根据结果执行代码，然后，使用(resultset.next() ，第二个结果集将总是由于第一个结果而跳过一行组

到目前为止，这是我的代码：

connection = DriverManager.getConnection(connection);
statement = (Statement) connection.createStatement();
resultset = statement.executeQuery(query);

String id = "0";
String idfromdb;

if(resultset.next() == false){
  System.out.println("table is empty")
  statement.executeUpdate(" INSERT INTO table VALUES ('value') ");
}

else{
  while(resultset.next()){
    idfromdb = resultset.getString("value")
    if(! idfromdb = id){
      System.out.println("no similar data has been found");
    }    
    else{
      System.out.println("similar data has been found");
    }
  }
}

Answer 1

当您调用resultset.next() ，它将获取一条记录，因此您需要调用previous（）来重置游标

完整代码：

boolean hasRecord = true;
if(!resultset.next()){// you are missing ) here
    System.out.println("table is empty")
    statement.executeUpdate(" INSERT INTO table VALUES ('value') ");
    hasRecord = false;
}
if(hasRecord){
   resultset.previous();//move to the previous record
}

Answer 2

更改为do-while循环：

if(resultset.next() == false{
    System.out.println("table is empty")
    statement.executeUpdate(" INSERT INTO table VALUES ('value') ");
} else {
  do  {
    idfromdb = resultset.getString("value")
    if(! idfromdb = id){
        System.out.println("no similar data has been found");
    }

    else{
        System.out.println("similar data has been found");
    }
  } while(resultset.next());
}

do-while在循环的底部而不是顶部评估其表达式。