How to get parameters from HTTP get url using Expression evaluator?

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I have created a microservice pipeline in streamsets. Upon making a get callout, i have to retrieve data from mysql depending on the parameters sent in the http get url using expression evaluator?

My url is supposed to be like this: Http://my.url.com:0191?param1=xyz&par...

I have to retrieve data based on param1 value and param2 value.

Also, how do I handle cases when the params send will be null?