如何在安卓 mysql 中获取单个列

Question

如何使用安卓获取单个列。 而不是从数据库中获取所有内容。 我只想获取 full_name="约翰" 的列。

DataParser.jave

public class DataParser  extends AsyncTask<Void,Void,Integer>{
Context c;
ListView lv;
String jsonData;
ProgressDialog pd;
ArrayList<Person> persons=new ArrayList<>();
public DataParser(Context c, ListView lv, String jsonData) {
    this.c = c;
    this.lv = lv;
    this.jsonData = jsonData;
}
@Override
protected void onPreExecute() {
    super.onPreExecute();
    pd=new ProgressDialog(c);
    pd.setTitle("Parse");
    pd.setMessage("Parsing...Please wait");
    pd.show();
}
@Override
protected Integer doInBackground(Void... params) {
    return this.parseData();
}
@Override
protected void onPostExecute(Integer result) {
    super.onPostExecute(result);
    pd.dismiss();
    if(result==0)
    {
        Toast.makeText(c,"Unable to parse",Toast.LENGTH_SHORT).show();
    }else {
        //CALL ADAPTER TO BIND DATA
        CustomAdapter adapter=new CustomAdapter(c,persons);
        lv.setAdapter(adapter);
    }
}

private int parseData()
{
    try {
        JSONArray ja=new JSONArray(jsonData);
        JSONObject jo=null;
        persons.clear();
        Person s=null;


        for(int i=0;i<ja.length();i++) {

            jo = ja.getJSONObject(i);

            int id = jo.getInt("id");
            String name = jo.getString("full_name");
            String sex = jo.getString("sex");
            String location = jo.getString("location");

            s = new Person();
            s.setId(id);
            s.setFull_name(name);
            s.setSex(sex);
            s.setLocation(location);

            persons.add(s);

        }
                return 1;


    } catch (JSONException e) {
        e.printStackTrace();
    }
    return 0;
}

}

下载器.java

public class Downloader extends AsyncTask<Void,Void,String> {
Context c;
String urlAddress;
ListView lv;
ProgressDialog pd;
public Downloader(Context c, String urlAddress, ListView lv) {
    this.c = c;
    this.urlAddress = urlAddress;
    this.lv = lv;
}
@Override
protected void onPreExecute() {
    super.onPreExecute();
    pd=new ProgressDialog(c);
    pd.setTitle("Fetch");
    pd.setMessage("Fetching....Please wait");
    pd.show();
}
@Override
protected String doInBackground(Void... params) {
    return this.downloadData();
}

@Override
protected void onPostExecute(String s) {
    super.onPostExecute(s);
    pd.dismiss();
    if(s==null)
    {
        Toast.makeText(c,"Unsuccessfull,Null 
    returned",Toast.LENGTH_SHORT).show();
    }else
    { 
        //CALL DATA PARSER TO PARSE
        DataParser parser=new DataParser(c,lv,s);
        parser.execute();
    }

}

private String downloadData()
{
    HttpURLConnection con=Connector.connect(urlAddress);
    if(con==null)
    {
        return null;
    }
    InputStream is=null;
    try {
        is=new BufferedInputStream(con.getInputStream());
        BufferedReader br=new BufferedReader(new InputStreamReader(is));
        String line=null;
        StringBuffer response=new StringBuffer();
        if(br != null)
        {
            while ((line=br.readLine()) != null)
            {
                response.append(line+"n");
            }
            br.close();
        }else
        {
            return null;
        }
        return response.toString();
    } catch (IOException e) {
        e.printStackTrace();
    }finally {
        if(is != null)
        {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    return null;
  }
 }

连接器.java

public class Connector {
public static HttpURLConnection connect(String urlAddress)
{
    try {
        URL url=new URL(urlAddress);
        HttpURLConnection con= (HttpURLConnection) url.openConnection();
        //SET PROPS
        con.setRequestMethod("GET");
        con.setConnectTimeout(20000);
        con.setReadTimeout(20000);
        con.setDoInput(true);
        return con;
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}

  }

人.java

public class Person {
    int id;
String full_name, sex, location;
public int getId() {
    return id;
}
public void setId(int id) {
    this.id = id;
}
public String getFull_name() {
    return full_name;
}
public void setFull_name(String full_name) {
    this.full_name = full_name;
}
public String getSex() {
    return sex;
}
public void setSex(String sex) {
    this.sex = sex;
}
public String getLocation() {
    return location;
}
public void setLocation(String location) {
    this.location = location;
}

}

自定义适配器.java

public class CustomAdapter extends BaseAdapter {
Context c;
ArrayList<Person> persons;
LayoutInflater inflater;
public CustomAdapter(Context c, ArrayList<Person> persons) {
    this.c = c;
    this.persons = persons;
    //INITIALIE
    inflater= (LayoutInflater) c.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
@Override
public int getCount() {
    return persons.size();
}
@Override
public Object getItem(int position) {
    return persons.get(position);
}
@Override
public long getItemId(int position) {
    return persons.get(position).getId();
}
@Override
public View getView(final int position, View convertView, ViewGroup parent) {
    if(convertView==null)
    {
        convertView=inflater.inflate(R.layout.model,parent,false);
    }
    TextView nameTxt= (TextView) convertView.findViewById(R.id.nameTxt);
    TextView sexTxt= (TextView) convertView.findViewById(R.id.propellantTxt);
    TextView locationTxt= (TextView) convertView.findViewById(R.id.descTxt);
    nameTxt.setText(persons.get(position).getFull_name());
    sexTxt.setText(persons.get(position).getSex());
    locationTxt.setText(persons.get(position).getLocation());
    //ITEM CLICKS
    convertView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(c,persons.get(position).getFull_name(),Toast.LENGTH_SHORT).show();
        }
    });
    return convertView;
}

}

服务器端 PHP 文件

<?php 
$host = 'localhost';
$username='root';
$pwd ='';
$db ='user';
$con=mysqli_connect($host,$username,$pwd,$db) or die('Unable to connect');
if(mysqli_connect_error($con))
{
echo "Failed to Connect to Database ".mysqli_connect_error();
}
$query=mysqli_query($con,"SELECT * FROM person");
if($query)
{
 while($row=mysqli_fetch_array($query))
{
$data[]=$row;
}
 print(json_encode($data));
}else
{
echo('Not Found ');
 }
 mysqli_close($con);
 ?>

主要活动.java

public class MainActivity extends AppCompatActivity {
String urlAddress="http://localhost/Android/includes/retrive.php";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    final ListView lv= (ListView) findViewById(R.id.lv);

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Downloader d=new Downloader(MainActivity.this,urlAddress,lv);
            d.execute();
        }
    });
}

}

在线获取代码，找不到获取单列的方法

Answer 1

看起来您使用的 json 来自 API 响应。 在这种情况下，如果要在SQL中执行此操作，则需要在服务器上创建新的api和SQL查询。 如果只想限制循环中显示的内容，请仅解析所需的部分。

for(int i=0;i<ja.length();i++) {

        jo = ja.getJSONObject(i);

        int id = jo.getInt("id");
        String name = jo.getString("full_name");
        s = new Person();
        s.setFull_name(name);
        persons.add(s);

    }

然后，您还需要更改活动中显示数据的逻辑，并且仅显示 person.name，因为其余属性将为 null。

我建议对使用 API 和解析 JSON 进行更多研究，因为您使用的代码似乎没有数据库或 SQL。