我的SQL语句不会从带有SQLite的android中的表中返回任何数据

Question

我有一个onClick按钮，当用户点击时，它会从数据库助手类调用一个方法来执行搜索。但由于某些原因它似乎没有用，请帮助我。 :)这是我的代码，其中包含按钮：

 public class LessonAlphabets extends Activity implements View.OnClickListener {

    Button btn;

    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      DatabaseHelper db = new DatabaseHelper(this);
      //some other unrelated code
    }

    public void onClick(View view) {
      btn = (Button) view;
      if(alphabetName.equals(alphabetHolder)){
          DatabaseHelper dbHelper = new DatabaseHelper(this);

          String text= dbHelper.getDescription(alphabetHolder);//this is the method to query
          dbHelper.close();
          textAlpha.setText(alphabetHolder + text);
    }

这是数据库助手类：

  public class DatabaseHelper extends SQLiteOpenHelper {
   //declarations and other methods
   //....
   //....

     public String getDescription(String s_name) {

       String data = "";

       String selectQuery = "SELECT " + KEY_DESC + " FROM " + TABLE_SIGN + " WHERE signName = '" + s_name + "'";

        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        if (cursor.moveToFirst()) {
            do {
                // get  the  data into array,or class variable
                data = cursor.getString(cursor.getColumnIndex(KEY_DESC));
            } while (cursor.moveToNext());

        }
        db.close();
        return data;
     }
   }

最后这里是数据库表的类（不知道这个问题是否重要）：

 public class Table_Sign {

        private int id;
        private String signName;
        private String signNameBM;
         private String desc;
         private String descBM;
         private int category;


public Table_Sign(){}

public Table_Sign(String signName, String signNameBM, String desc, String descBM, int category) {
    super();
    this.signName = signName;
    this.signNameBM = signNameBM;
    this.desc = desc;
    this.descBM = descBM;
    this.category = category;
}

//------- getters & setters

// getting ID
public int getId(){
    return this.id;
}
// setting id
public void setId(int id){
    this.id = id;
}

// getting signName
public String getSignName(){
    return this.signName;
}
// setting signName
public void setSignName(String signName){
    this.signName = signName;
}

// getting signNameBM
public String getSignNameBM(){
    return this.signNameBM;
}
// setting signNameBM
public void setSignNameBM(String signNameBM){
    this.signNameBM = signNameBM;
}

// getting desc
public String getDesc(){
    return this.desc;
}
// setting desc
public void setDesc(String desc){
    this.desc = desc;
}

// getting descBM
public String getDescBM(){
    return this.descBM;
}
// setting descBM
public void setDescBM(String descBM){
    this.descBM = descBM;
}

// getting category
public int getCategory(){
    return this.category;
}
// setting category
public void setCategory(int category){
    this.category = category;
}

//------- getters & setters

@Override
public String toString() {
    return "Table_Sign [id=" + id + ", signName=" + signName + ", signNameBM=" + signNameBM + ", desc=" + desc + ", descBM=" + descBM
            + ", category=" + category + "]";
}

}