Details are described in the document “Grid-Like Access to Aggregated Data”, sections 4 and 8.1: https://www.oasis-open.org/committees/document.php?document_id=62593&wg_abbrev=odata Excerpt from relevant section 8.1: Extend the expressions that can be passed to the orderby transformation by an optional suffix that, if specified, determines the position of undefined values of properties that have been aggregated away: With orderby( <orderbyItem> undefined first ) undefined values of the orderby item come before any defined value. The order among the undefined values is undefined. With orderby( <orderbyItem> undefined last ) undefined values of the orderby item come after any defined value. The order among the undefined values is undefined.

As per 2018-6-7, closing with no action. We can already support this by putting expressions in the orderby:   orderby(Continent,isdefined(Country) desc,Country desc) orderby(Continent,isdefined(Country) asc,Country desc) orderby(Continent,Country eq null asc,Country desc)