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。