This definitely depends on the employer and your geographical location; for example, in California, clauses like this are not legal except under certain conditions: https://leginfo.legislature.ca.gov/faces/codes_displaySectio...
If you work for a big enough tech company, like, say, Google or Facebook, it's likely that a lot of things will "relate...to their business." And, note, too, that there's no requirement that you actually know that it relates to a part of your employer's business.
OK if your a semi pro musician but a generic developer sie gig not so much.