Sql – How to select case when a string is not equal a value then make another field blank

sql

I have some columns like" OrderfieldNumber, Purchaser, Item. I'd like to create a statement in which:
case when the purchaser is not 'ABC', 'DEF', 'GHK' then the OrderFieldNumber is blank. My purpose is just want to show OrderFieldNumber with those buyers only, I want to ignore other buyers without using Filter function. I tried:

case
when purchaser <> 'ABC' then OrderFieldNum = ' '
when purchaser <> 'DEF' then OrderFieldNum = ' '
when purchaser <> 'GHK' then OrderFieldNum = ' '
ELSE purchaser end as FilteredOrderField

But it doesn't seem to work out.

Best Solution

case when purchaser not in ('ABC','DEF','GHK') 
     then ' '
     else OrderFieldNum 
end as FilteredOrderField