Left join causing headache - sql

Question

I am troubleshooting a process that is running a very long time, This select statemet runs for over 30 minutes, when i comment out various parts of it, it runs very fast. i have put a * next to the Tables that appear to be causing the issue , the one with two ** has a clustered index on Empid and ChangeDate(these columns are not next to each other) not sure if that matters, also none of these tables has more than 3000 rows. I am hoping there is some simple tweak hat needs to be made. Is there something i should look at in source tables? any suggestions would be appreciated. i have googled 'multiple out joins in single statement' with no luck. For troubleshooting purposes i put the with(nolock) in.
select
je.EmpID,
max(med.ChangeDate) as Max_Medical_ChangeDate,
max(den.ChangeDate) as Max_Dental_ChangeDate,
max(k.ChangeDate) as Max_401K_ChangeDate,
max(ltd.ChangeDate) as Max_LTD_ChangeDate,
max(std.ChangeDate) as Max_STD_ChangeDate,
max(life.ChangeDate) as Max_Life_ChangeDate,
max(sal.ChangeDate) as Max_Salary_ChangeDate,
max(ltdexe.ChangeDate) as Max_LTDEXE_ChangeDate
max(accid.ChangeDate) as Max_Accid_ChangeDate,
max(cancr.ChangeDate) as Max_Cancr_ChangeDate,
max(hosp.ChangeDate) as Max_Hosp_ChangeDate
from Judge_Emp je WITH(NOLOCK)
left join Medical med on je.EmpID = med.EmpID
left join Dental den on med.EmpID = den.EmpID
left join R401K k on den.EmpID = k.EmpID
left join STDIns std on k.EmpID = std.EmpID
left join LTDIns ltd on std.EmpID = ltd.EmpID
* left join LTDEXEIns ltdexe on ltd.EmpID = ltdexe.EmpID
** left join LifeIns life on ltdexe.EmpID = life.EmpID
* left join Salary sal on life.EmpID = sal.EmpID
left join AF_Accid accid on accid.EmpID=sal.EmpID
left join AF_Cancr cancr on cancr.EmpID=accid.EmpID
left join AF_Hosp hosp on hosp.EmpID=cancr.EmpID
group by je.EmpID
Thanks

Answer 1

This is too long for a comment.
If you are using left outer joins, your join conditions should refer to the first table in the chain. For instance:
from Judge_Emp je WITH(NOLOCK)
left join Medical med on je.EmpID = med.EmpID
left join Dental den on je.EmpID = den.EmpID
left join R401K k on je.EmpID = k.EmpID . . .
Rather than:
from Judge_Emp je WITH(NOLOCK)
left join Medical med on je.EmpID = med.EmpID
left join Dental den on med.EmpID = den.EmpID
left join R401K k on den.EmpID = k.EmpID . . .
The reasons is that med.EmptID could be NULL, which would cause the join to fail. (It doesn't make a difference for inner joins).
This probably doesn't affect the performance of the query, though. My guess is that these have multiple matching rows and are generating a Cartesian product for each customer.