How to connect my android application to an external database [duplicate]

First of all, you will need a connection class, I use this in my projects with help with php:

public class Conexao {

public static String postDados(String urlUsuario, String parametrosUsuario) {
    URL url;
    HttpURLConnection connection = null;

    try {

        url = new URL(urlUsuario);
        connection = (HttpURLConnection) url.openConnection();

        connection.setRequestMethod("POST");

        connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded;charset=utf-8");

        connection.setRequestProperty("Content-Lenght", "" + Integer.toString(parametrosUsuario.getBytes().length));

        connection.setRequestProperty("Content-Language", "pt-BR");

        connection.setUseCaches(false);
        connection.setDoInput(true);
        connection.setDoOutput(true);

        OutputStreamWriter outPutStream = new OutputStreamWriter(connection.getOutputStream(), "utf-8");
        outPutStream.write(parametrosUsuario);
        outPutStream.flush();
        outPutStream.close();

        InputStream inputStream = connection.getInputStream();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "utf-8"));

        String linha;
        StringBuffer resposta = new StringBuffer();

        while((linha = bufferedReader.readLine()) != null) {
         resposta.append(linha);
            resposta.append('\r');
        }

        bufferedReader.close();

        return resposta.toString();

    } catch (Exception erro) {

        return  null;
    } finally {

        if(connection != null) {
            connection.disconnect();
        }
    }
}
}

Ready, now to use it you do this:

public class Main extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    String texto = "123";
    url = "url_do_seu_arquivo_php"; //Caso queira um get, só colocar na url
    //  ?variavel=valor

    //Parâmentros por post
    parametros = "texto=" + texto;

    new SolicitaDados().execute(url);

 }

 private class SolicitaDados extends AsyncTask<String, Void, String> {
    @Override
    protected  String doInBackground(String... urls) {

        return Conexao.postDados(urls[0], parametros);

    }

    @Override
    protected void onPostExecute(String resultado) {
        //String resultado tem o retorno


    }
 }
 }

You can use Firebase for this, in addition to the database it offers you the possibility of user authentication, pushNotifications and several other tools, and besides it has a very generous free plan, so you do not have to spend nothing at first. In addition to being cross-platform.

MySql I think that is not possible. There is a google tool called FireBase. It is not a database but it can serve what you need.