java - (初学者)SQLite 查询不会使用 MoveToNext 进入 if-loop

Question

我整个早上都在 StackOverflow 上尝试自学如何在 Android 应用程序上对我上传的 SQLite 数据库进行查询。我尝试过原始查询和直接查询，但到目前为止都没有奏效。这是代码:

public Product getProduct(int id) {
    db = this.getReadableDatabase();


//        new String[] { COLUMN_ID, COLUMN_PRODUCT_NAME, COLUMN_SIZE, COLUMN_HP,
//        COLUMN_CATEGORY, COLUMN_POWER_TYPE, COLUMN_EXP_PROOF, COLUMN_RPM,
//                COLUMN_PSI, COLUMN_GPM, COLUMN_PRICE},

//        Cursor cursor = db.query(
//                TABLE_NAME,
//                null,
//                COLUMN_ID + " = ?",
//                new String[] { Integer.toString(id)},
//                null, null, null, null);

    String strquery = "SELECT * FROM " + TABLE_NAME + " WHERE " + COLUMN_ID + "=" + id;
    Cursor cursor = db.rawQuery(strquery, null);

    Product product = new Product();


    if (cursor.moveToFirst()) {
        product.setID(cursor.getInt(0));
        product.setName(cursor.getString(1));
        product.setSize(cursor.getDouble(2));
        product.setHp(cursor.getDouble(3));
        product.setCategory(cursor.getString(4));
        product.setPowerType(cursor.getString(5));
        product.setExplosionProof(cursor.getInt(6));
        product.setRPM(cursor.getInt(7));
        product.setBypassPSI(cursor.getInt(8));
        product.setGPM(cursor.getInt(9));
        product.setPrice(cursor.getDouble(10));
        cursor.close();
    }

    db.close();
    return product;
}

注释掉的字段是我之前尝试过的查询，但都不起作用。

这是我的常量:

private static final String TABLE_NAME = "Products";
private static final String COLUMN_ID = "'_id'";

/image/5YiZr.png^ 这是我正在使用的数据库屏幕截图的链接。

我调试它时的问题是它永远不会进入if循环，并且mCount始终为-1。我已经做了很多研究，但我仍然对此感到沮丧。谢谢

[编辑}

这是我的数据库代码

public class SQLiteHelper extends SQLiteOpenHelper {

private SQLiteHelper sInstance;

private static final int DATABASE_VERSION = 1;
private static final String DATABASE_NAME = "ProductList.db";
private SQLiteDatabase db;
private final Context myContext;
private String databasePath = "";


public SQLiteHelper(Context context){
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.myContext = context;
    databasePath = context.getDatabasePath(DATABASE_NAME).getPath();
}


private static final String TABLE_NAME = "Products";
private static final String COLUMN_ID = "'_id'";
private static final String COLUMN_PRODUCT_NAME = "'Name'";
private static final String COLUMN_SIZE = "'Size'";
private static final String COLUMN_HP = "'HP'";
private static final String COLUMN_CATEGORY = "'Category'";
private static final String COLUMN_POWER_TYPE = "'PowerType'";
private static final String COLUMN_EXP_PROOF = "'ExpProof'";
private static final String COLUMN_RPM = "'RPM'";
private static final String COLUMN_PSI = "'PSI'";
private static final String COLUMN_GPM = "'GPM'";
private static final String COLUMN_PRICE = "'Price'";

public void createDataBase() throws IOException {
    boolean dbExists = checkDataBase();
    if (dbExists) {
    } else {
        this.getReadableDatabase();
        try {
            copyDataBase();
        } catch (IOException e) {
            throw new Error("Error making database");
        }
    }
}

private boolean checkDataBase() {
    SQLiteDatabase check = null;
    try {
        String path = databasePath + DATABASE_NAME;
        check = SQLiteDatabase.openDatabase(path, null, SQLiteDatabase.OPEN_READONLY);
    } catch (SQLiteException e) {
        // database doesn't exist
    }

    if (check != null) {
        check.close();
    }

    return check != null;
}

private void copyDataBase() throws IOException{
    InputStream myInput = myContext.getAssets().open(DATABASE_NAME);

    String outFileName = databasePath + DATABASE_NAME;

    OutputStream myOutput = new FileOutputStream(outFileName);

    byte[] buffer = new byte[1024];
    int length;
    while ((length=myInput.read(buffer))>0) {
        myOutput.write(buffer, 0, length);
    }

    // close streams
    myOutput.flush();
    myOutput.close();
    myInput.close();
}



public void openDataBase() throws SQLException {

    //Open the database
    String myPath = databasePath;
    db = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}

@Override
public synchronized void close() {

    if(db != null)
        db.close();
    super.close();

}

@Override
public void onCreate(SQLiteDatabase db) {

}

Answer 1

请检查您的这部分代码。

public void openDataBase() throws SQLException {
    //Open the database
    String myPath = databasePath;
    db = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}

这里您只是使用databasePath，我发现您已将其设置为从构造函数中的context.getDabasePath 获得的值，但查看您拥有的其他方法看起来像是在创建新的数据库路径:

private void copyDataBase() throws IOException{
    InputStream myInput = myContext.getAssets().open(DATABASE_NAME);

    String outFileName = databasePath + DATABASE_NAME;

    OutputStream myOutput = new FileOutputStream(outFileName);

    byte[] buffer = new byte[1024];
    int length;
    while ((length=myInput.read(buffer))>0) {
        myOutput.write(buffer, 0, length);
    }

    // close streams
    myOutput.flush();
    myOutput.close();
    myInput.close();
}

不确定这是否是由于从源代码复制/粘贴造成的，但这看起来像是导致您丢失数据的原因。