Tim Dietrich's SuiteQL Query Tool is an excellent way to learn, create and execute SuiteQL. And, queries created in the SuiteQL Query Tool can be used directly in a NetSheet.io spreadsheet. The SuiteQL Query Tool can be downloaded here. Tim also has many informative articles on SuiteQL on his site.
"The SuiteQL Query Tool is a NetSuite native application that can be used to run SQL queries directly within a NetSuite instance. With this free Suitelet, you can create and run SuiteQL queries, save queries to your File Cabinet, load queries from a shared hosted query library, and much more." ~ Tim Dietrich