Question

I have two tables in my database – rooms and devices. Each room can have many devices. I want to make an expandable listview with room name as group and devices name and state as children (on or off).

Can any one give me a simple idea because I am kind of new with Android and all tutorials are hard to make

This is my database:

public void onCreate(SQLiteDatabase db) {
        db.execSQL("create table if not exists rooms (
                                    id_room integer primary key autoincrement, "
                + "name_room text not null" 
                + ");");    
        db.execSQL("create table if not exists devices (
                                    id_device integer primary key autoincrement, "
                + "name_device text not null," 
                + "state_device text not null,"
                + "id_room references rooms"
                + ");");
    }

Answer 1

Following is a very simple, no frills expandable list implementation.

You need two methods in your db helper class to gather the cursors you will need.

public Cursor fetchGroup() {
    String query = "SELECT * FROM rooms"
    return mDb.rawQuery(query, null);
}

public Cursor fetchChildren(String room) {
    String query = "SELECT * FROM devices WHERE id_room = '" + room + "'";
    return mDb.rawQuery(query, null);
}

Then you need to set up your adapter (in your activity):

public class MyExpandableListAdapter extends SimpleCursorTreeAdapter {
    public MyExpandableListAdapter(Cursor cursor, Context context,int groupLayout, 
        int childLayout, String[] groupFrom, int[] groupTo, String[] childrenFrom, 
        int[] childrenTo) {
            super(context, cursor, groupLayout, groupFrom, groupTo,
                  childLayout, childrenFrom, childrenTo);
        }
    }

    @Override
    protected Cursor getChildrenCursor(Cursor groupCursor) {
        Cursor childCursor = mDbHelper.fetchChildren(groupCursor.getString(groupCursor.getColumnIndex("id_room"));            
        getActivity().startManagingCursor(childCursor);
        childCursor.moveToFirst();
        return childCursor;
    }
}

And finally call the adapter and set it to your list (in your activity):

private void fillData() {
    mGroupsCursor = mDbHelper.fetchGroup();
    getActivity().startManagingCursor(mGroupsCursor);
    mGroupsCursor.moveToFirst();

    ExpandableListView elv = (ExpandableListView) getActivity().findViewById(android.R.id.list);

    mAdapter = new MyExpandableListAdapter(mGroupsCursor, getActivity(),
        R.layout.rowlayout_expgroup,                     // Your row layout for a group
        R.layout.rowlayout_itemlist_exp,                 // Your row layout for a child
        new String[] { "id_room" },                      // Field(s) to use from group cursor
        new int[] { android.R.id.room },                 // Widget ids to put group data into
        new String[] { "name_device", "state_device" },  // Field(s) to use from child cursors
        new int[] { R.id.device, R.id.state });          // Widget ids to put child data into

        lv.setAdapter(mAdapter);                         // set the list adapter.
    }
}

Hope this helps!

EDIT

It should all come together like so:

public class List_Exp extends Activity {
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mDbHelper = new YourDB(getActivity());
        mDbHelper.open();
        fillData();
    }

    private void fillData() { 
        // set list adapter here
    }

    public class MyExpandableListAdapter extends SimpleCursorTreeAdapter {
        // Your adapter
    }
}

EDIT 2

To catch clicks, set listeners in you activity:

lv.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {
    @Override
    public boolean onChildClick(ExpandableListView parent, View v,
        int groupPosition, int childPosition, long id) {
        // Your child click code here
        return true;
    }
});

lv.setOnGroupClickListener(new ExpandableListView.OnGroupClickListener() {
    @Override
    public boolean onGroupClick(ExpandableListView parent, View v,
        int groupPosition, int groupPosition, long id) {
        // Your group click code here
        return true;
    }
});