Select with case in a "WHERE"

I'm trying to run a select statement and want the where clause to use different criteria depending on the value of a variable.  When I run the below I get an error
     Msg 102, Level 15, State 1, Line 9
     Incorrect syntax near '='.

Can I not do that in a where clause?


declare @Itemtype varchar(1)
set @Itemtype='1'

select 'pop30110',iv1.*
from pop30110 iv1
join dynamics_ext.dbo.tmpinvalid tmp on tmp.itemnmbr=iv1.itemnmbr
--where tmp.itemtype=@itemtype
--where tmp.itemtype ='1'
where (case when @Itemtype='' then tmp.itemtype ='1' else tmp.itemtype=@itemtype end )
jdr0606Asked:
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Brian CroweDatabase AdministratorCommented:
select 'pop30110',iv1.*
from pop30110 iv1
join dynamics_ext.dbo.tmpinvalid tmp on tmp.itemnmbr=iv1.itemnmbr
WHERE (@itemtype = '' AND tmp.itemtype = '1')
   OR (@itemtype <> '' AND tmp.itemtype = @itemtype)
0

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Scott PletcherSenior DBACommented:
Much better query optimization by just making sure that @itemtype is never '':

declare @Itemtype varchar(1)
 set @Itemtype='1'
if @Itemtype = ''
    set @Itemtype = '1'

select ...
where tmp.itemtype = @itemtype
0
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