Nested Repeat General How-To

It’s been a while since I’ve done one of these. I think it was back in the HTML5 data-binding days, lol. I need to do some nested repeat type regions, where I have something like multiple categories, and rows within those categories, but I don’t want to see the category repeated. Something like:
Category1
Item 1
Item 2
Item 3
Category2
Item 1
Item 2
Category3
Item1

Instead of this:
Category1 Item1
Category1 Item2
Category1 Item3
Category2 Item1
etc,

Is this done client side, or server side with the sql grouping?
Sorry if this has been covered, I didn’t find it.

Thanks,
Chris