android - db.rawQuery SELECT SUM 中的游标返回 null : how to catch?

Question

我正在运行以下查询，

Cursor c = db.rawQuery("SELECT SUM(surveycount) FROM surveyDB WHERE species = " + "'" + szSpecies + "' AND location = " + "'" + szLocation + "'", null); 

创建表时将数据库列“surveycount”声明为 INT。下面的代码工作正常，除非没有返回具有 szSpecies 和 szLocation 目标字符串值的记录，此时 szSum 显示为 null。

if (c.moveToFirst()) {
            szSum = c.getString(0);
            c.close();}
            else{
                //do nothing
                Toast.makeText(getActivity().getApplicationContext(),"It was NULL",Toast.LENGTH_LONG).show();
            }
Toast.makeText(getActivity().getApplicationContext(),"Surveycount SUM = "+szSum+" .",Toast.LENGTH_LONG).show();

如何捕获这个错误的光标输出？

Answer 1

首先检查游标是否不为空，然后查看cursor.getcount()是否大于0，然后尝试从游标中检索值。

更新后的代码如下:

Cursor c = db.rawQuery("SELECT SUM(surveycount) FROM surveyDB WHERE species = " + "'" + szSpecies + "' AND location = " + "'" + szLocation + "'", null); 
  szSum="";

if(c!=null && c.getCount()>0)
{
            c.moveToFirst()
            szSum = c.getString(0);
            c.close();
} else{
   //do nothing
   Toast.makeText(getActivity().getApplicationContext(),"It was NULL",Toast.LENGTH_LONG).show();
}

if(!szSum.equals(""))
    Toast.makeText(getActivity().getApplicationContext(),"Surveycount SUM = "+szSum+" .",Toast.LENGTH_LONG).show();

getCount():返回游标中的行数