The two functional groups characteristic of sugars are

Monosaccharides can be further classified by their most highly oxidized functional group. Sugars are chemically carbohydrates. They are polyhydroxy aldehydes ketones and their condensation products. Aldoses bear a terminal aldehyde or -CHO group while ketoses have an internal ketone or -CO group.
Thus, they possess two functional groups, i.e. carbonyl and hydroxyl.