signal语句用于显式地发出一个错误并强制调用处理程序。
The SIGNAL statement is used to explicitly raise an error and force the invocation of a handler.
这个参数强制使用特殊的错误代码，以使该问题区别于其他问题。
It is imperative to use a unique error code for this parameter to distinguish this problem from others.
这些文件是样例XML文件，可用于测试我们的程序而无须强制数据库错误或使用Optim Performance Manager。
These are sample XML files that can be used to test our program without having to force database errors or use Optim Performance Manager.
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