Here’s my top 3 favorite uses for CROSS APPLY and OUTER APPLY:
APPLY is fantastic for calling table valued functions. I didn’t include questions about those in the quiz, simply for the purposes of keeping the code simple, and because I wanted the quiz to be about thinking through how apply works — but it’s still the #1 use.
Another thing I might use it for is when a query needs a correlated subquery — somewhat like an inline function.
And I also like it for queries that have a calculation that needs to be done and which is referenced in multiple columns in the query, or perhaps also in a predicate and the select. You can perform the computation once in the apply and then reference it multiple times. That way if you have to change the formula later on, you only have to change it in once place, plus I find it’s easier to read in some cases.
Item number three is something I learned from an older blog post Kendra wrote and regularly use APPLY for this purpose.