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

cross apply sometimes enables you to do things that you cannot do with inner join.

Example (a syntax error):

select F.* from sys.objects O  
inner join dbo.myTableFun(O.name) F   
on F.schema_id= O.schema_id

This is a syntax error, because, when used with inner join, table functions can only take variables or constants as parameters. (I.e., the table function parameter cannot depend on another table's column.)

However:

select F.* from sys.objects O  
cross apply ( select * from dbo.myTableFun(O.name) ) F  
where F.schema_id= O.schema_id

This is legal.

Edit: Or alternatively, shorter syntax: (by ErikE)

select F.* from sys.objects O  
cross apply dbo.myTableFun(O.name) F
where F.schema_id= O.schema_id

Edit:

Note: Informix 12.10 xC2+ has Lateral Derived Tables and Postgresql (9.3+) has Lateral Subqueries which can be used to a similar effect.

Answer 2

I guess it should be readability ;)

CROSS APPLY will be somewhat unique for people reading to tell them that a UDF is being used which will be applied to each row from the table on the left.

Ofcourse, there are other limitations where a CROSS APPLY is better used than JOIN which other friends have posted above.