consider provide more examples

Thanks for the project! I am just playing with the parser to get used to it. However, I could not figure out how we get to the components that were parsed from the AST parser. I followed an example as described in [here](https://github.com/ballista-compute/sqlparser-rs/#example). Since only a few examples are [given](https://github.com/ballista-compute/sqlparser-rs/tree/main/examples), I think it is a bit off to understand and extend this library by following only those examples. It's better to add more examples. :)

首先阅读 README 示例和 examples 目录中的现有文件。确定有关访问从 AST 解析出的组件的指导缺失之处，然后添加有针对性的示例，使这种用法和扩展路径更加清晰；完成的标准是示例能够解释超出现有覆盖范围的相关 parser 结果。

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