Jia,

you can use pre built queries and you can create queries. i believe Robin has some code to get you started here: http://ryehle.wordpress.com/2012/06/28/custom-build-result-pruner/

See Sam's answer https://jazz.net/forum/questions/84612/workitem-state-change-programatically?page=1&focusedAnswerId=84627#84627 for the state change. You can also search the SDK for examples. I found the code there.

I have used this kind of code to query:

        IWorkItemClient  workItemService = (IWorkItemClient) targetRepository
            .getClientLibrary(IWorkItemClient.class);
        IQueryClient queryClient = workItemService.getQueryClient();
        IAuditableClient auditableClient = (IAuditableClient)targetRepository.getClientLibrary(IAuditableClient.class);
        IQueryableAttribute attribute=QueryableAttributes.getFactory(IWorkItem.ITEM_TYPE).findAttribute(targetProjectArea, IWorkItem.PROJECT_AREA_PROPERTY, auditableClient, monitor);
        Expression inProjectArea= new AttributeExpression(attribute,AttributeOperation.EQUALS, targetProjectArea);
        IQueryResult<IResolvedResult<IWorkItem>> results=queryClient.getResolvedExpressionResults(targetProjectArea, inProjectArea,IWorkItem.SMALL_PROFILE);
        results.setLimit(Integer.MAX_VALUE);

        while (results.hasNext(monitor)) {
            IResolvedResult<IWorkItem> resresult = (IResolvedResult<IWorkItem>) results.next(monitor);
            IWorkItem item = resresult.getItem();

Not sure if there is a clause for querying a state group or state, I guess there is.