您设计的派生类必须有一个名为 definition（可以不修改此名）的嵌套的模板类/结构。
The derived class that you design must have a nested template class/structure named definition (you may not change this name). The definition class has the following properties
这也就是说在不修改任何表信息情况下就可以修改模式的用法。
This means that modifications to schema usage can be made without modifying any table information.
这种类型的操作不修改资源，因此可以调用多次并获得相同的结果。
This type of operation doesn't modify the resource and can therefore be called many times with the same results.
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