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DAOpublic 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);    }