JDBC一对多联接与多项选择

Question

I am currently writing a program for which I need to select an entity from the database which has multiple one to many relationships. In this program the main object a Route is made of many Route Points has many Comments and many Reports. This means if i want to select a Route from the database along with its Route Points, Comments and Reports as a result of the join I am going to get multiple rows back with duplicate information in.

As far as I am aware this will mean I will have to loop on each row and keep a set of if I have already "seen" this RoutePoint, Report or Comment. As a Route can have a large number of Route Points I imagine this would be quite slow as well as been quite complicated and messy. I have included below what this would look like.

HashSet<String> commentIds = new HashSet<String>();
HashSet<String> routePointIds = new HashSet<String>();
HashSet<String> reportIds = new HashSet<String>();

Route route = null;
        while (resultSet.next()) {
            String commentId = resultSet.getString("commentId");
            String routePointId = resultSet.getString("routePointId");
            String reportId = resultSet.getString("reportId");

            if(route == null){
                // create route from result set
            }

            if(!commentIds.contains(commentId)){
                // create comment object from result set
                // add to route object

                commentIds.add(commentId);
            }

            if(!routePointIds.contains(routePointId)){
                // create route point object from result set
                // add to route object

                routePointIds.add(routePointId);
            }

            if(!reportIds.contains(reportId)){
                // create comment object from result set
                // add to route

                reportIds.add(reportId);
            }

        }

My question is, is there a simpler way of processing a query with multiple one to many joins in JDBC?

Another way I have thought of approaching this is to do multiple selects. One for each table rather than joining. I am not sure if this is a suitable way to do this as it would mean querying the database multiple times rather than just the once.

Thanks for your replies.

Answer 1

Query for distinct rows for each independent result set. Since you are only querying for one route's data, I'm assuming you are not storing this in a cache or memory, and will need to perform this action any time you need the route data. It will more efficient to use an optimized query, and avoid java logic for each row returned to weed out duplicate data.