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 selecting over large data sets if you are partitioning. (Paging comes to mind)

I also know that CROSS APPLY doesn't require a UDF as the right-table.

In most INNER JOIN queries (one-to-many relationships), I could rewrite them to use CROSS APPLY, but they always give me equivalent execution plans.

Can anyone give me a good example of when CROSS APPLY makes a difference in those cases where INNER JOIN will work as well?

---

Edit:

Here's a trivial example, where the execution plans are exactly the same. (Show me one where they differ and where cross apply is faster/more efficient)

create table Company (
    companyId int identity(1,1)
,   companyName varchar(100)
,   zipcode varchar(10) 
,   constraint PK_Company primary key (companyId)
)
GO

create table Person (
    personId int identity(1,1)
,   personName varchar(100)
,   companyId int
,   constraint FK_Person_CompanyId foreign key (companyId) references dbo.Company(companyId)
,   constraint PK_Person primary key (personId)
)
GO

insert Company
select 'ABC Company', '19808' union
select 'XYZ Company', '08534' union
select '123 Company', '10016'


insert Person
select 'Alan', 1 union
select 'Bobby', 1 union
select 'Chris', 1 union
select 'Xavier', 2 union
select 'Yoshi', 2 union
select 'Zambrano', 2 union
select 'Player 1', 3 union
select 'Player 2', 3 union
select 'Player 3', 3 


/* using CROSS APPLY */
select *
from Person p
cross apply (
    select *
    from Company c
    where p.companyid = c.companyId
) Czip

/* the equivalent query using INNER JOIN */
select *
from Person p
inner join Company c on p.companyid = c.companyId

Answer 1

The essence of the APPLY operator is to allow correlation between left and right side of the operator in the FROM clause.

In contrast to JOIN, the correlation between inputs is not allowed.

Speaking about correlation in APPLY operator, I mean on the right hand side we can put:

• a derived table - as a correlated subquery with an alias
• a table valued function - a conceptual view with parameters, where the parameter can refer to the left side

Both can return multiple columns and rows.

Answer 2

Cross apply can be used to replace subquery's where you need a column of the subquery

subquery

select * from person p where
p.companyId in(select c.companyId from company c where c.companyname like '%yyy%')

here i won't be able to select the columns of company table so, using cross apply

select P.*,T.CompanyName
from Person p
cross apply (
    select *
    from Company C
    where p.companyid = c.companyId and c.CompanyName like '%yyy%'
) T

Answer 3

here is an example when CROSS APPLY makes a huge difference with performance:

Using CROSS APPLY to optimize joins on BETWEEN conditions

Note that besides replacing inner joins you can also reuse code such as truncating dates without paying performance penalty for involing scalar UDFs, for example: Calculating third Wednesday of the month with inline UDFs

Answer 4

Here is an article that explains it all, with their performance difference and usage over JOINS.

SQL Server CROSS APPLY and OUTER APPLY over JOINS

As suggested in this article, there is no performance difference between them for normal join operations (INNER AND CROSS).

The usage difference arrives when you have to do a query like this:

CREATE FUNCTION dbo.fn_GetAllEmployeeOfADepartment(@DeptID AS INT)  
RETURNS TABLE 
AS 
RETURN 
   ( 
   SELECT * FROM Employee E 
   WHERE E.DepartmentID = @DeptID 
   ) 
GO 
SELECT * FROM Department D 
CROSS APPLY dbo.fn_GetAllEmployeeOfADepartment(D.DepartmentID)

That is, when you have to relate with function. This cannot be done using INNER JOIN, which would give you the error "The multi-part identifier "D.DepartmentID" could not be bound." Here the value is passed to the function as each row is read. Sounds cool to me. :)

Answer 5

This has already been answered very well technically, but let me give a concrete example of how it's extremely useful:

Lets say you have two tables, Customer and Order. Customers have many Orders.

I want to create a view that gives me details about customers, and the most recent order they've made. With just JOINS, this would require some self-joins and aggregation which isn't pretty. But with Cross Apply, its super easy:

SELECT *
FROM Customer
CROSS APPLY (
  SELECT TOP 1 *
  FROM Order
  WHERE Order.CustomerId = Customer.CustomerId
  ORDER BY OrderDate DESC
) T

Answer 6

This is perhaps an old question, but I still love the power of CROSS APPLY to simplify the re-use of logic and to provide a "chaining" mechanism for results.

I've provided a SQL Fiddle below which shows a simple example of how you can use CROSS APPLY to perform complex logical operations on your data set without things getting at all messy. It's not hard to extrapolate from here more complex calculations.

http://sqlfiddle.com/#!3/23862/2