In SSRS 2008, how are parameters passed down to the DatSet Query... or can they?

In SSRS 2008, how are parameters passed down to the DatSet Query... or can they?

I have a simple question. I think this is pretty straight-forward. If necessary, I can include screen shots of what I mean.

Here is the basic format of the report. I have a SSRS report made on Visual Studio 2017. it has two DataSets. It has one Tablix and one Bar Chart that is loaded with from the DataSets. I have one Parameter defined for the report. The parameter a Date/Time data type set to allow nulls and I name it rundate. (I will use is in a data driven subscription, but for now that is beyond the scope of my question here)

Ffor each dataset, I have gone into the Datset Properties and added a parameter with a Parameter Name of rundate and Parameter Value [@rundate]. I think it would be great if I make this somehow a variable in the Query for the Dataset. I assume that this is exactly what this is supposed to be used for. How do I declare it in the SQL query.

I said that this was a pretty straight forward question. it should be easily answered and it will save time in digging around the internet for the answer. So far, all I have found is how to set up filters. This is not exactly the same sort of thing. But, if necessary, I guess I could resort to using filters but it will take some changes in the query. And it would be a mess.

I got this query working in SSRS (I think) https://photos.app.goo.gl/GSAd6yKqHf5Mw3sf7 Now I just need to test for '@rundate' equals null. How would I do that in SQL? https://photos.app.goo.gl/XEaAjeh4kfsgdNro9

I have no problem when users talk about non-HeidiSQL topics here. But I guess noone can help you here.