If the app doesn't want to do anything special for this ID, it returns NULL, and the window function senses this and news up the appropriate default object.
如果程序不需要这个ID，则返回NULL，窗口函数根据返回值创建恰当的默认对象。
If the app doesn't want to do anything special for this ID, it returns NULL, and the window function senses this and news up the appropriate default object.
如果程序不需要这个ID，则返回NULL，窗口函数根据返回值创建恰当的默认对象。
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