always empty cursor is returned

The part of code that returns cursor is as follows

 public Cursor getAllRecords() {
        SharedPreferences pref = mContext.getSharedPreferences("lin", 0);
        String a = pref.getString("KEY", null);          // getting String
        Log.d("test=====>", a);
        SQLiteDatabase databaseX = getReadableDatabase();

        return databaseX.rawQuery("SELECT * FROM " + TABLE_NAME + " WHERE " + HISTORY_COLUMN_NAME + " = '" + a + "'", null);

    }