Android - 没有这样的表：现有表中的材料

Question

我有两个表Materias（纪律）和Grade（网格）。首先我从表中调用活动Materias并插入一些行，然后我调用Grade活动。它有一个来自 的参考列Materias，但是当我内部加入这些表时，我收到致命异常，说这条消息：

SQLiteException: no such table: Materias (code 1): , while compiling: SELECT g.*, c.Descricao FROM Grades g INNER JOIN Materias m ON  m.id = g.materia"

我的材料DAO

public class MateriaDAO extends SQLiteOpenHelper {

    public final String tabela = "Materias";

    public MateriaDAO(Context context) {

        super(context, "Materias", null, 2);

    }

    @Override

    public void onCreate(SQLiteDatabase db) {

        String comando = "CREATE TABLE " + tabela + " (";

        comando += "id INTEGER PRIMARY KEY,";

        comando += "Descricao TEXT,";

        comando += "Comentario TEXT,";

        comando += "Qtd_Aulas INTEGER,";

        comando += "Qtd_Faltas INTEGER)";

        db.execSQL(comando);

    }

    @Override

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

        String sql = "DROP TABLE IF EXISTS " + tabela;

        db.execSQL(sql);

        onCreate(db);

    }

    public void insert(Materia o) {    

        try{

            ContentValues cv = new ContentValues();

            cv.put("Descricao", o.getDescricao());

            cv.put("Comentario", o.getComentario());

            cv.put("Qtd_Aulas", o.getQtdAula());

            cv.put("Qtd_Faltas", o.getQtdAula());

            getWritableDatabase().insert(tabela, null, cv);

        } catch (Exception ex) {

            throw ex;

        }

    }

    public void update(Materia o) {

        ContentValues cv = new ContentValues();

        cv.put("Descricao", o.getDescricao());

        cv.put("Comentario", o.getComentario());

        cv.put("Qtd_Aulas", o.getQtdAula());

        cv.put("Qtd_Faltas", o.getQtdAula());

        String[] args = {o.getID() + ""};

        getWritableDatabase().update(tabela, cv, "ID=?", args);

    }