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。