When should I use cross apply over inner join?

Question

What is the main purpose of using CROSS APPLY?

I have read (vaguely, through posts on the Internet) that cross apply can be more efficient when selectin

Answer 1

It seems to me that CROSS APPLY can fill a certain gap when working with calculated fields in complex/nested queries, and make them simpler and more readable.

Simple example: you have a DoB and you want to present multiple age-related fields that will also rely on other data sources (such as employment), like Age, AgeGroup, AgeAtHiring, MinimumRetirementDate, etc. for use in your end-user application (Excel PivotTables, for example).

Options are limited and rarely elegant:

• JOIN subqueries cannot introduce new values in the dataset based on data in the parent query (it must stand on its own).

• UDFs are neat, but slow as they tend to prevent parallel operations. And being a separate entity can be a good (less code) or a bad (where is the code) thing.

• Junction tables. Sometimes they can work, but soon enough you're joining subqueries with tons of UNIONs. Big mess.

• Create yet another single-purpose view, assuming your calculations don't require data obtained mid-way through your main query.

• Intermediary tables. Yes... that usually works, and often a good option as they can be indexed and fast, but performance can also drop due to to UPDATE statements not being parallel and not allowing to cascade formulas (reuse results) to update several fields within the same statement. And sometimes you'd just prefer to do things in one pass.

• Nesting queries. Yes at any point you can put parenthesis on your entire query and use it as a subquery upon which you can manipulate source data and calculated fields alike. But you can only do this so much before it gets ugly. Very ugly.

• Repeating code. What is the greatest value of 3 long (CASE...ELSE...END) statements? That's gonna be readable!

  • Tell your clients to calculate the damn things themselves.

Did I miss something? Probably, so feel free to comment. But hey, CROSS APPLY is like a godsend in such situations: you just add a simple CROSS APPLY (select tbl.value + 1 as someFormula) as crossTbl and voilà! Your new field is now ready for use practically like it had always been there in your source data.

Values introduced through CROSS APPLY can...

• be used to create one or multiple calculated fields without adding performance, complexity or readability issues to the mix
• like with JOINs, several subsequent CROSS APPLY statements can refer to themselves: CROSS APPLY (select crossTbl.someFormula + 1 as someMoreFormula) as crossTbl2
• you can use values introduced by a CROSS APPLY in subsequent JOIN conditions
• As a bonus, there's the Table-valued function aspect

Dang, there's nothing they can't do!