Android对数据库表的一个约定：每张表都应该至少有_id这列

       Android对数据库表有一个约定。就是每张表都应该至少有_id这列。ListView在使用CursorAdapter及其子类适配 cursor的时候，会默认的获取 _id 这列的值，如果你建的表没有 _id这列或者你的cursor中没有_id这列（查询时的projection中没有_id）就报错了。所以使用CursorAdapter及其子类的时候一定要使查询时的projection包含_id。CursorAdapter中相关代码如下：

1.注释  
/** 
 * Adapter that exposes data from a {@link android.database.Cursor Cursor} to a  
 * {@link android.widget.ListView ListView} widget. The Cursor must include  
 * a column named "_id" or this class will not work. 
 */  

2.构造方法  
    public CursorAdapter(Context context, Cursor c) {  
        init(context, c, true);  
    }  
public CursorAdapter(Context context, Cursor c, boolean autoRequery) {  
        init(context, c, autoRequery);  
    }  
  
    protected void init(Context context, Cursor c, boolean autoRequery) {  
        boolean cursorPresent = c != null;  
        mAutoRequery = autoRequery;  
        mCursor = c;  
        mDataValid = cursorPresent;  
        mContext = context;  
        mRowIDColumn = cursorPresent ? c.getColumnIndexOrThrow("_id") : -1;  
        mChangeObserver = new ChangeObserver();  
        if (cursorPresent) {  
            c.registerContentObserver(mChangeObserver);  
            c.registerDataSetObserver(mDataSetObserver);  
        }  
    }  

3.public void changeCursor(Cursor cursor) {  
        if (cursor == mCursor) {  
            return;  
        }  
        if (mCursor != null) {  
            mCursor.unregisterContentObserver(mChangeObserver);  
            mCursor.unregisterDataSetObserver(mDataSetObserver);  
            mCursor.close();  
        }  
        mCursor = cursor;  
        if (cursor != null) {  
            cursor.registerContentObserver(mChangeObserver);  
            cursor.registerDataSetObserver(mDataSetObserver);  
            mRowIDColumn = cursor.getColumnIndexOrThrow("_id");  
            mDataValid = true;  
            // notify the observers about the new cursor  
            notifyDataSetChanged();  
        } else {  
            mRowIDColumn = -1;  
            mDataValid = false;  
            // notify the observers about the lack of a data set  
            notifyDataSetInvalidated();  
        }  
    }

其中cursor.getColumnIndexOrThrow(String columnName)如下：

    /** 
     * Returns the zero-based index for the given column name, or throws 
     * {@link IllegalArgumentException} if the column doesn't exist. If you're not sure if 
     * a column will exist or not use {@link #getColumnIndex(String)} and check for -1, which 
     * is more efficient than catching the exceptions. 
     * 
     * @param columnName the name of the target column. 
     * @return the zero-based column index for the given column name 
     * @see #getColumnIndex(String) 
     * @throws IllegalArgumentException if the column does not exist 
     */  
    int getColumnIndexOrThrow(String columnName) throws IllegalArgumentException;

所以只要Cursor不为空，Cursor中必须存在_id列，CursorAdapter及其子类才能正常工作。

转自：https://blog.csdn.net/spare_h/article/details/6716810