A sleeve for a shelving unit has two halves joined together to form two support tabs extending outward from opposing sides of the sleeve as the sleeve clamps around a post having grooves into which a rib on the sleeve fits to hold the parts relative to the post. The support tabs have a retaining flange offset from the sleeve. A first collar part fastened to a shelf fits over half the sleeve and fits into that offset so that a slot in the first collar part fits over one of the support tabs to clamp them to the post and support the first collar part and shelf. The retaining flange keeps the collar part from moving off the support tab. A second collar part with a second slot may fit over the other support tabs to support a second shelf or to further clamp the sleeve to the post.