filteredSubClass = Keys@DeleteMissing[
   EntityClass["MinorPlanet", "DwarfPlanet"]["Radius", "EntityAssociation"]
   ];

EntityValue[filteredSubClass, {"Entity", "Radius", "OrbitPeriod"}]

In this example filteredSubClass targets only "Radius", does not care if "OrbitPeriod" is Missing.

I think I understand what you mean, but let's take things slower. Isn't your example redundant because Radius and OrbitPeriod are Missing for the same entries?

EntityClass["MinorPlanet", "DwarfPlanet"][{"Entity", "Radius", "Density"}] // Column

Thanks for your responses! In your example - you've put Radius and Density, not Radius and OrbitPeriod,

But in general I was using DwarfPlanets as an example because it's a small data set and just using a few properties. My larger goal was wider dataset with more properties where i just wanted to filter out stuff that didn't have a radius.

Hi

Thanks to both of you for explaining the error of my ways. In the first example - DeleteMissing, I was totally unaware i could do this with the entities, and also found out i could stack the properties together - which is great. However, I wasn't able to extend DeleteMissing for that scenario:

DeleteMissing[EntityClass["MinorPlanet", "DwarfPlanet"][{"Entity", "Radius",  "Density"}], 1, 2]

would also filter out missing Densities. However after playing around i found i could use DeleteCases with a pattern so something like:

DeleteCases[ EntityClass["MinorPlanet", "DwarfPlanet"][{"Entity", "Radius",   "Density"}], {_, _Missing, _}]

Thanks to Christopher for pointing out what i was missing - great stuff, appreciate it.

Working versions of your filter designs:

shouldFilter1[planet_] := 
 Head[MinorPlanetData[planet, "Radius"]] === Missing
filteredDwarves1 := DeleteCases[dwarves, p_ /; shouldFilter1[p]]

shouldFilter2[planet_] := 
 MatchQ[MinorPlanetData[planet, "Radius"], _Missing]
filteredDwarves2 := DeleteCases[dwarves, p_ /; shouldFilter2[p]]

shouldFilter3[planet_] := 
 MinorPlanetData[planet, "Radius"] > Quantity[0.0, "miles"]
filteredDwarves3 := Cases[dwarves, p_ /; shouldFilter3[p]]

What you where having trouble with were some subtleties. Double = is Equal, like an equation. Triple = is SameQ, a test for lexicographical matching. Two ways to scan for the head Missing, one is to isolate using Head then test it with SameQ; another is to use pattern matching, like MatchQ. Filter #3 worked, but it was a positive value Quantity filter when used with DeleteCases.