如何从Room SQLite数据库中检索数据？

Question

所以我根据YouTube上的教程创建了一个房间数据库。我有两个栏day and likes,每个栏都有int.目前，我已经用三行数据手动填充了数据库。

下面是手动填充数据库的代码：

@Database(entities = Recovery.class, version = 1, exportSchema = false)
public abstract class RecoveryDatabase extends RoomDatabase {

private static RecoveryDatabase INSTANCE;
public abstract RecoveryDao recoveryDao();

// Singleton
public static RecoveryDatabase getInstance(final Context context) {
    if (INSTANCE == null) {
        synchronized (RecoveryDatabase.class) {
            if (INSTANCE == null) {
                INSTANCE = Room.databaseBuilder(context.getApplicationContext(), RecoveryDatabase.class, "recovery_database").fallbackToDestructiveMigration().addCallback(roomCallback).build();
            }
        }
    }
    return INSTANCE;
}

private static RecoveryDatabase.Callback roomCallback = new RoomDatabase.Callback() {
    @Override
    public void onCreate(@NonNull SupportSQLiteDatabase db) {
        super.onCreate(db);
        new PopulateDbAsyncTask(INSTANCE).execute();
    }
};

private static class PopulateDbAsyncTask extends AsyncTask<Void, Void, Void> {
    private RecoveryDao recoveryDao;

    private PopulateDbAsyncTask(RecoveryDatabase db) {
        recoveryDao = db.recoveryDao();
    }

    @Override
    protected Void doInBackground(Void... voids) {
        // First input is the day and the second is the num of likes for that day
        recoveryDao.insert(new Recovery(6,1));
        recoveryDao.insert(new Recovery(7,2));
        recoveryDao.insert(new Recovery(8,3));
        return null;
    }
}

在我的Dao类中，我当前有Insert、Update、deleteAll和getall...方法。这里是道：

@Dao
public interface RecoveryDao {

@Insert
void insert(Recovery recovery);

@Update
void update(Recovery recovery);


@Query("DELETE FROM recovery_table")
void deleteAllRecovery();

// Sends data from ascending day to the call. (1,2,3,4 ....)
@Query("SELECT * FROM RECOVERY_TABLE ORDER BY day ASC")
LiveData<List<Recovery>> getAllRecovery();

}

现在，我要从Room数据库中检索基于当天的数据。所以我想要第6天的likes数，它是1。我想检索数据并将其存储到MainActivity中的int变量中。

Answer 1

尝试这个查询，您需要在参数中传递日期。

@Query("SELECT * FROM RECOVERY_TABLE WHERE day=:day ORDER BY day ASC ")
LiveData<List<Recovery>> getRecoveryWithDay(int day);

这将为您提供具有匹配日期的恢复列表，如果需要，您可以将返回类型从LiveData 更改为list

如果你发现任何错误，请告诉我。