Want to hook up a sink in my basement. I have a 2"pvc riser close by, can I hook to it above the cement.

First, investigate what that riser is. It could be a sewer ejector discharge pipe or ejector vent pipe and that would not be suitable for sink drain. Other than that, you should be able to connect to it with your sink drain

Hope that helps

Milo

2 inch would be too small for a house drain. Where does this riser go? Sounds like a vent or, as Milo said, a vent or connection to an ejector pit. My ejector is 2" and if same as yours, must be a dedicated piping only.