相关疑难解决方法(0)

我试图通过使用异步查询提供CursorLoader了SimpleCursorTreeAdapter

这是我Fragment实现的类CursorLoader

public class GroupsListFragment extends ExpandableListFragment implements
  LoaderManager.LoaderCallbacks<Cursor> {

  private final String DEBUG_TAG = getClass().getSimpleName().toString();      

  private static final String[] CONTACTS_PROJECTION = new String[] {
    ContactsContract.Contacts._ID,
    ContactsContract.Contacts.DISPLAY_NAME };  

  private static final String[] GROUPS_SUMMARY_PROJECTION = new String[] {
    ContactsContract.Groups.TITLE, ContactsContract.Groups._ID,
    ContactsContract.Groups.SUMMARY_COUNT,
    ContactsContract.Groups.ACCOUNT_NAME,
    ContactsContract.Groups.ACCOUNT_TYPE,
    ContactsContract.Groups.DATA_SET };

  GroupsAdapter mAdapter;

  @Override
  public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    populateContactList();

    getLoaderManager().initLoader(-1, null, this);
  } 

  public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    // This is called when a new Loader needs …

我确信这有一个非常简单的答案,但我似乎无法找到它(不确定要搜索什么!).查询的标准计数/组可能如下所示:

SELECT COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 

这按预期工作,如果没有找到行则返回0.然而:

SELECT COUNT(`t2`.`name`) 
FROM `table_1` `t1` 
    LEFT JOIN `table_2` `t2` ON `t1`.`key_id` = `t2`.`key_id` 
WHERE `t1`.`another_column` = 123 
GROUP BY `t1`.`any_col` 

仅当table_1中至少有一行时才有效,如果有零行则无法返回空结果集.我真的希望这回到0!有谁启发我这个？如果您在伦敦,可以提供啤酒;-)