从sql数据库一个​​细胞 [英] select one cell from sql database

问题描述

我想一个简单的SQL命令wihtin我的Andr​​oid的应用程序，以获得选定人员的年龄：

I'm trying a simple SQL Command wihtin my Android-App, to get the age of a selected Person:

public int getAge(){
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery("SELECT * FROM persons WHERE name =? " + MainActivity.selectedPerson.getText().toString(), null);
    int age = cursor.getInt(3);               // column with ages
    cursor.close();
    db.close();
    return age;
}

但是当我运行我的应用程序，它，当我调用函数getAge（）崩溃。我收到以下错误：

But when I run my app, it crashes when I call the function getAge(). I get the following Error:

SQLiteException: no such column: Max: , while compiling: SELECT * FROM persons WHERE name = Max

我不明白这一点。有在表中的名称为最大。我究竟做错了什么？先谢谢了。

I don't get it. There is the name "Max" in the table. What am I doing wrong? Thanks in advance.

编辑2：
随着这一个：

Edit 2: With this one:

Cursor cursor = db.rawQuery("SELECT name FROM persons WHERE name = '" + MainActivity.selectedPerson.getText().toString() + "'", null);

我得到一个不同的错误：

I get a different error:

08-27 19:43:47.573: E/AndroidRuntime(6161): android.database.CursorIndexOutOfBoundsException: Index -1 requested, with a size of 1

这是什么意思？

推荐答案

您应该考虑使用 rawQuery的）的 selectionArgs两个参数（至prevent SQL注入攻击的：

You should consider using the selectionArgs parameter of rawQuery() to prevent SQL Injection Attacks:

Cursor cursor = db.rawQuery("SELECT * FROM persons WHERE name = ?", new String[] { MainActivity.selectedPerson.getText().toString() });

此外，你只需要的有一个的列，这样，而不是用 * ，你应该只选择一列中选择他们所有的资源浪费：

Also you only need one column so rather than wasting resources by selecting them all with *, you should just select the one column:

Cursor cursor = db.rawQuery("SELECT age FROM persons WHERE name = ?", new String[] { MainActivity.selectedPerson.getText().toString() });

希望帮助！

总之它应该是这样的：

public int getAge(){
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery("SELECT age FROM persons WHERE name = ?", new String[] { MainActivity.selectedPerson.getText().toString() });

    int age;
    // Check if you have a valid result and move to the first row to read it
    if(cursor.moveToFirst())
        age = cursor.getInt(0);
    // Prevent a crash if there is no data for this name
    else
        age = 0;

    cursor.close();
    db.close();
    return age;
}

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