if object_id('test') is not null drop table test
go
create table test(PersonID int,AttnIDs varchar(20))
go
insert into test
select 18,'13 10' union all
select 15,'16 15' union all
select 15,'9'
go
declare @xml xml,@str varchar(max)    
select @str=isnull(@str,'')+'<x>'+convert(varchar(5),PersonID)+'</x><x>'+replace(replace(AttnIDs,convert(varchar(5),PersonID),''),' ','</x><x>')+'</x>' from test
select @xml=convert(xml,@str)

;with cte as
(
    select  N.v.value('.','varchar(10)') ID
    from @xml.nodes('/x') N(v)
)

select ID,count(ID) cnt from cte
where ID<>''
group by ID

/*

(3 行受影响)
ID         cnt
---------- -----------
10         1
13         1
15         2
16         1
18         1
9          1

(6 行受影响)

*/