Finishing off my 'make folders from database' class

The below is a class for making local folders from database entries where each folder has a name, id and parent-id. I have put it together as best I can but do not know enough to finish it off.

I need to "just grab the folder with id 0 and start building your Files on the disk, using folder.getChildren() as a convenient way to move down the tree then just mkdirs()" as told to me in another post but I do not understand how and where to do it. Please help

public class Loop {

public static void main(String[] args) {
    int PID = 0;
    int RepoID = 1;
    Connection con = null;
    String url = "jdbc:mysql://localhost/document_manager";
    String user = "root";
    String password = "Pa55w0rd";

    try {
        con = DriverManager.getConnection(url, user, password);
    } catch (SQLException e) {
        e.printStackTrace();
    }
    Map<Integer,Folder> data = new HashMap<Integer,Folder>(); 
    while( PID < 50 )
    {
    try {            
     Statement st = con.createStatement();
        ResultSet result = st.executeQuery("SELECT name, category_id, parent_id FROM categories WHERE parent_id = '"+PID+"' AND repository_id = '"+RepoID+"'");
        while (result.next ())
        {

            String FolderName = result.getString ("name");
            String FolderId = result.getString ("category_id");
            String ParentId = result.getString ("parent_id");
            int intFolderId = Integer.parseInt(FolderId);
            int intParentId = Integer.parseInt(ParentId);
            System.out.println( FolderId+" "+FolderName+" "+ParentId );
            //intFolderId = Integer.valueOf(FolderId);
            /开发者_StackOverflow/intParentId = Integer.valueOf(FolderId);
            Folder newFolder = new Folder(FolderName, intFolderId, intParentId);
            data.put(newFolder.getId(), newFolder);
            }

    } catch (SQLException ex) {
        System.out.println(ex.getMessage());
    }

     PID++;

}
    for(Folder folder : data.values()) {
        int parentId = folder.getParentFolderId();
        Folder parentFolder = data.get(parentId);
        if(parentFolder != null)
            parentFolder.addChildFolder(folder);
        }
} 
   }

Basically reading hierachies from the database is not trivial, but if you read one level per query that should be doable.

What you need to do is the following:

• select all folders that have no parent, those are your root folders
• create the folders if they don't exist already
• repeat the following until you don't get any more results from the db
• select all folders whose parent id is the id of the folders read in the previous iteration (or the parents)
• assign the read subfolders to their parents based on the parent id
• create the read subfolders in their parents

This would allow you to use a breadth-first approach, i.e. you read one level of folders per iteration and map the children to the parents using parent_id.

As an alternative you could read the children for a single parent and iterate over the hierarchy in a depth-first manner.