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

Well I am not sure if this qualifies as a reason to use Cross Apply versus Inner Join, but this query was answered for me in a Forum Post using Cross Apply, so I am not sure if there is an equalivent method using Inner Join:

Create PROCEDURE [dbo].[Message_FindHighestMatches]

-- Declare the Topical Neighborhood
@TopicalNeighborhood nchar(255)

AS BEGIN

-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON

Create table  #temp
(
    MessageID         int,
    Subjects          nchar(255),
    SubjectsCount    int
)

Insert into #temp Select MessageID, Subjects, SubjectsCount From Message

Select Top 20 MessageID, Subjects, SubjectsCount,
    (t.cnt * 100)/t3.inputvalues as MatchPercentage

From #temp 

cross apply (select count(*) as cnt from dbo.Split(Subjects,',') as t1
             join dbo.Split(@TopicalNeighborhood,',') as t2
             on t1.value = t2.value) as t
cross apply (select count(*) as inputValues from dbo.Split(@TopicalNeighborhood,',')) as t3

Order By MatchPercentage desc

drop table #temp

END