declare @department table (dID int, department varchar(20))
insert @department
select 800, 'tiles' union all
select 900, 'plumbing' union all
select 1000, 'paint'

declare @supplier table (sID varchar(5), supplier varchar(20))
insert @supplier
select 'G000', 'CTM' union all
select 'K000', 'caroma' union all
select 'Z000', 'dulux'

declare @Associations table (aID int, dID int, sID varchar(5))
insert @associations
select 7001, 800, 'G000' union all
select 7002, 900, 'K000' union all
select 7003, 1000, 'Z000' union all
select 7004, 800, 'Z000' union all
select 7005, 900, 'G000'

declare @spenditure table (aID int, total int)
insert @spenditure
select 7001, 5000 union all
select 7002, 8500 union all
select 7003, 3200 union all
select 7004, 4650 union all
select 7005, 2100


select s.sID
       ,max(e.total) AS total
       ,max(ca.department) as department --min / max doesn't matter because of group by in cross apply
from   @supplier s
join   @associations a 
       on a.sid = s.sid
join   @spenditure e
       on e.aID = a.aID
cross apply (
       select top 1 d1.department
       from   @supplier s1
       join   @associations a1
              on a1.sID = s1.sID
       join   @spenditure e1
              on e1.aID = a1.aID
       join   @department d1
              on d1.dID = a1.dID
       where  a1.sID = s.sID
       group by d1.department
       order by sum(e1.total) desc
       ) ca
GROUP by s.sID
ORDER BY s.sID 

OUTPUT:
sID   total       department
----- ----------- --------------------
G000  5000        tiles
K000  8500        plumbing
Z000  4650        tiles