如何在BaseColumns中使用_COUNT个 [英] How to use _COUNT in BaseColumns

问题描述

我一直在阅读BaseColumns]( https://Android中的developer.android.com/reference/android/provider/BaseColumns.html )来帮助构建数据库架构.

I've been reading up on BaseColumns](https://developer.android.com/reference/android/provider/BaseColumns.html) in Android to help structure my database schema.

我知道_ID是您必须创建自己的行的唯一标识符:

I know that _ID is a unique identifier for the row that you have to create yourself:

protected static final String SQL_CREATE = "CREATE TABLE " + TABLE_NAME + "( " + 
        _ID + " INTEGER PRIMARY KEY AUTOINCREMENT" +  ...;

我还读到_COUNT用于引用表中的行数.

I also read that _COUNT is used to refer to the number of rows in a table.

但是，当我尝试使用_COUNT时，出现错误.这是我尝试过的:

However, when I tried using _COUNT, I got an error. Here is what I tried:

SQLiteDatabase db = TimetableDbHelper.getInstance(context).getReadableDatabase();
Cursor cursor = db.query(
        SubjectsSchema.TABLE_NAME,
        new String[] {SubjectsSchema._COUNT},
        null, null, null, null, null);
cursor.moveToFirst();
int count = cursor.getInt(cursor.getColumnIndex(SubjectsSchema._COUNT));
cursor.close();
return count;

我不确定这是否是使用它的正确方法，但是出现了这个错误:

I'm not sure whether or not this is the correct way to use it, but I got this error:

android.database.sqlite.SQLiteException:无此类列:_count(代码1):，而在编译时:从主题中选择_count个

android.database.sqlite.SQLiteException: no such column: _count (code 1): , while compiling: SELECT _count FROM subjects

我应该如何使用_COUNT?

推荐答案

在数据库中，_id或_count没有什么特别的.

In the database, there is nothing special about either _id or _count.

当表被定义为具有这样的列时，或者当查询显式计算它时，您的查询将返回_id或_count列.

Your queries return an _id or _count column when the table is defined to have such a column, or when the query explicitly computes it.

Android框架的许多对象都希望游标具有唯一的_id列，因此有很多表对其进行定义.

Many objects of the Android framework expect a cursor to have a unique _id column, so many tables define it.

在大多数地方，预计不会出现_count，因此通常不实施.并且如果确实需要它，则可以使用子查询简单地计算出它，如下所示:

In most places, the _count is not expected to be present, so it is usually not implemented. And if it is actually needed, it can simply be computed with a subquery, like this:

SELECT _id,
       [other fields],
       (SELECT COUNT(*) FROM MyTable) AS _count
FROM MyTable
WHERE ...

如果要查找自己的表的大小，则不需要使用_count名称.您可以执行类似SELECT COUNT(*) FROM subjects的查询，或者甚至更简单地使用

If you want to find out the size of your own table, you are not required to use the _count name; you can execute a query like SELECT COUNT(*) FROM subjects, or, even simpler, use a helper function that does this for you.

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