Basically Workflow is a tool which needs a hands-on experience. Although a background in programming concepts and experience in OO development is necessary I think. And there is very little actual coding unless you build your own modules, concepts such as loops and boolean logic are needed but done with visual objects. There is no inherent need to write SQL queries but if you wish to store custom data or retrieve data you will need to provide your own queries. So the need for SQL depends what you want to do.