Returning back to that list that I talked about in a previous post we had

ANYELEMENT,CREATEREF, DEREF, IS OF, KEY, MULTISET, NAVIGATE, OFTYPE, OVERLAPS, Projection (including here as I've not yet figured out what it is :-)), REF, ROW , SELECT (including here because it has some weirdness like VALUE), SET, TREAT, USING

For this post, let's take a look at;

KEY

KEY selects us the key portion of an entity. So, if we have something like [order details] in Northwind then selecting KEY is going to give us a tuple ( orderid, productid ). So, we can run a query with something like;

"select value(key(od)) from NorthwindContext.Order_Details as od"

 and that'll give us;

and so on.