Question

Can the below query be converted to using an inner join instead of the where IN subquery?

select i.*, it.*
from ItemTypes it
    inner join Items i on (i.itemTypeID = it.ItemTypeID)
where i.itemID IN (......)

I might have 100 results returned in the sub-query and I want to avoid having that in a sub-query.

Answer 1

You could use the WITH common table expression (CTE):

;with SelectedItems (selectedIDs) as
(
    select distinct itemID from [...] where [...]
)
select i.*, it.*
from ItemTypes it
inner join Items i on i.itemTypeID = it.ItemTypeID
inner join SelectedItems on selectedIDs = i.itemID

You could also define a temporary #Table with an index to hold the selections:

create table #SelectedItems
(
    selItemID int primary key clustered
)
insert into #SelectedItems (selItemID)
select [...]

select i.*, it.*
from ItemTypes it
inner join Items i on i.itemTypeID = it.ItemTypeID
inner join #SelectedItems si on si.selItemID = i.itemID

drop table #SelectedItems

But whatever you do, be careful to measure performance before and after any change.