Difference betwwen the execution od these Queries

hi,

I have "order" and "busunit" tables

Order table has 417135 rows.

Case 1:

select * from ord
where idBusUnit in (select idBusUnit from busunit where idBusUnit < 500)

It is taking 46 seconds to get the results


Case 2:

select * from ord
inner join busunit
on
ord.idBusUnit = busunit.idBusUnit
and busunit.idBusUnit < 500

It is taking 1 min 23 seconds to get the results


Case 3:

select * from ord
inner join busunit
on
ord.idBusUnit = busunit.idBusUnit
Where busunit.idBusUnit < 500

It is taking 1 min 46 seconds to get the results

Case 4:

declare @tbl table (idBusUnit int)

insert into @tbl
select idBusUnit from busunit where idBusUnit < 500

select * from ord
inner join @tbl tbl
on
ord.idBusUnit = tbl.idBusUnit

It is taking 1 min 46 seconds to get the results
Next time I run the same query it is taking 1.09 sec


Please tell me in each case steps how we are getting the results.
And if I am running it for the second time, then why the time taken is less than
the first one.

Thanks & Regards,
butu