java jdbc访问多个结果集

Question

I have the following structure:

List -->List_Participant -->Participant

so a list may contain several participants.I try to read this in java:

        stat = con.createStatement();
        ResultSet rs = stat.executeQuery("select * from list;");
        // get the informations about the bracket sheet
        while (rs.next()) {
          string name = rs.getString("Name");
          ResultSet rs2 = stat.executeQuery("select * from List_Participant where name= '"+name+"';"); 

            while (rs2.next()) {
               // get the participants

            }
            rs2.close();
        }
        rs.close();

But this does not work. I don't receive an exception nor any other output. I suggest opening a second resultset will close the first one because since I do the first resultset, store the data in an arraylist and close it and afterwards the second it would work, but that leads to a poor performance because I have to search always in the arraylist.

What might be a better solution?

Edit: Solution is to make a Join, my current try:

 select * from List_participant 
INNER JOIN List ON List.name = List_participant.List 
INNER JOIN participant ON List_participant.participant =participant.ROWID;

How do I adress the columns now, since they might have the same name?

Answer 1

You can try using two different Statement instances for each query. See the JavaDoc for java.sql.Statement . The following example shows the principle.

Statement statement1 = connection.createStatement();
Statement statement2 = connection.createStatement();

ResultSet resultSet1 = statement1.executeQuery("select * from list");
while (resultSet1.next()) {
    String name = resultSet1.getString("Name");
        
    ResultSet resultSet2 = statement2.executeQuery(
        "select * from List_Participant where name= '" + name + "'");
    while (resultSet2.next()) {
        // get the participants
    }
}

BUT: This is not standard usage of JDBC or SQL for good reasons. It deprives the database of any optimization possibility and moves to much data between the DB and your app for no good reason (See the comments of JohnSkeet and BalusC).

Better use appropriate JOIN s in your one and only statement. This can be optimized by the DB:

SELECT lp.* FROM list l JOIN List_Participant lp ON l.name = lp.name

Add any filters/conditions you like to minimize the data retrieved.

Answer 2

Here is the reason why you can't have two ResultSet opened from the same Statement

ResultSet javadoc :

A ResultSet object is automatically closed when the Statement object that generated it is closed, re-executed , or used to retrieve the next result from a sequence of multiple results.

So basicly, a Statement can only give you one ResultSet at a time, so you loose the first result when you execute the second query.

Solution :

• Using one instance of Statement per ResultSet needed.
• merging the queries to only have one

Answer 3

Nested query ? Something like Select * from List_Participant where name in (select name from List); This can be made to work for your third tabel as well.