Question

我必须创建一个应用程序，为此我需要创建一个数据库。我能够创建新条目，但我不知道如何读取数据库以获取存储的数据并将其放入ListView。

public class DatabaseHelper extends SQLiteOpenHelper {

private static final String DATABASE_NAME="students.db";
private static final String TABLE_NAME="student_details";
private static final String COL_1="ID";
private static final String COL_2="NAME";
private static final String COL_3="SURNAME";
private static final String COL_4="MARKS";
private static final int DATABASE_VERSION=1;

public DatabaseHelper (Context context)
{
    super(context, DATABASE_NAME, null, DATABASE_VERSION);

}



@Override
public void onCreate(SQLiteDatabase db) {


db.execSQL("create table " + TABLE_NAME + "(ID INTEGER PRIMARY KEY AUTOINCREMENT,NAME TEXT,SURNAME TEXT,MARKS INTEGER)");

    Log.d("info", "TABLE WAS CREATED");


}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    db.execSQL("DROP TABLE IF EXIST " + TABLE_NAME);
    onCreate(db);
    Log.d("Info","TABLE WAS UPDATED");

}

public boolean insertdata(String name,String surname,String marks)
{

    SQLiteDatabase db= this.getWritableDatabase();

    ContentValues contentValues= new ContentValues();
    contentValues.put(COL_2,name);
    contentValues.put(COL_3, surname);
    contentValues.put(COL_4, marks);

    long result=db.insert(TABLE_NAME,null,contentValues);
    if (result==-1)
        return false;
    else
        return true;

}
public Cursor getallData()
{
    SQLiteDatabase db= this.getWritableDatabase();

    Cursor res=db.rawQuery("SELECT * FROM "+TABLE_NAME,null);
    return res;

}
public boolean updateData(String id,String name,String surname,String marks) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues contentValues = new ContentValues();
   // contentValues.put(COL_1, id);
           contentValues.put(COL_2, name);
    contentValues.put(COL_3, surname);
    contentValues.put(COL_4, marks);

    db.update(TABLE_NAME, contentValues, COL_1 + " =? ", new String[]{id});
    return true;

}
public Cursor getData(int id){
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor res =  db.rawQuery( "select * from student_details where ID="+id+"", null );
    if(res != null && res.moveToFirst()) {
        return res;
    }
    return res;
}
public int deleteData(String id)
{
    SQLiteDatabase db = this.getWritableDatabase();
    return db.delete(TABLE_NAME, COL_1 + " =?", new String[]{id});

}

 public class DisplayData extends Activity {

    DatabaseHelper myDB;
    ListView list;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.data_display);
        list= (ListView) findViewById(R.id.listView);
        myDB= new DatabaseHelper(this);
        //Cursor res = myDB.getallData();
        String[] from={"ID","NAME","SURNAME","MARKS"};

        int[] to={R.id.textView11,R.id.textView13,R.id.textView15 };
        Cursor cursor=myDB.getallData();
       // allData();
        SimpleCursorAdapter adapter= new SimpleCursorAdapter(this,R.layout.custom_data_display,cursor,from,to);
        list.setAdapter(adapter);
        // adapter= new ArrayAdapter(this,R.layout.custom_data_display,)


    }

这是我得到的错误：

com.example.ky.studentmarks E/AndroidRuntime﹕ FATAL EXCEPTION: main
        Process: com.example.aeiltech.studentmarks, PID: 23469
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ky.studentmarks/com.example.aeiltech.studentmarks.DisplayData}: java.lang.IllegalArgumentException: column '_id' does not exist
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2436)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2498)
                at android.app.ActivityThread.access$900(ActivityThread.java:179)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1324)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:146)
                at android.app.ActivityThread.main(ActivityThread.java:5641)
                at java.lang.reflect.Method.invokeNative(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:515)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1288)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1104)
                at dalvik.system.NativeStart.main(Native Method)

Answer 1

在进入您不了解的编码之前，您应该先尝试一下教程。
官方：
1. schema。
类似：
2. http://developer.android.com/training/basics/data-storage/databases.html。
好教程：
3. Android column '_id' does not exist?。
您的问题似乎是您没有SimpleCursor在数据库中需要的那一列（＆＃34; _id＆＃34;）。检查（2）您遇到的这个问题，并且为了解如何解决问题，请看（3）。
快乐的编码。

Answer 2

将表格中的var allClasses = GetProjectItems(Context.ActiveProject.ProjectItems).Where(v => v.Name.Contains(".cs")); // check for .cs extension on each foreach (var c in allClasses) { var eles = c.FileCodeModel; if (eles == null) continue; foreach (var ele in eles.CodeElements) { if (ele is EnvDTE.CodeNamespace) { var ns = ele as EnvDTE.CodeNamespace; // run through classes foreach (var property in ns.Members) { var member = property as CodeType; if (member == null) continue; foreach (var d in member.Bases) { var dClass = d as CodeClass; if (dClass == null) continue; var name = member.Name; if (dClass.Name == "DataMigrationImpl") { yield return new Migration(member); } } } } } }更改为id。此外，您需要将_id字段声明为SimpleCursorAdapter adapter类的字段，而不是DisplayData中的局部变量。

Answer 3

尝试像这样创建数据库：

 private static final String DATABASE_CREATE = "create table "
        + TABLE_NAME + "(" + COL_1 +
        " integer primary key autoincrement, "  +
        COL_2 + " text not null, "  + COL_3 + " text not null, " + COL_4 + " integer);";

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(DATABASE_CREATE);
}

同样在public Cursor getallData()中，请务必在res.moveToFirst()之前加return res。

看看voggela tutorial因为你需要一个DAO类来从数据库中访问数据而不直接使用它。

SimpleCursorAdapter将数据从数据库获取到ListView

3 个答案: