Here we are again

select individual_ref from contact where individual_ref in (190999,191000,191001,191002)

returns

191002
191002

there are 2 matching records. Thats correct

select individual_ref from individual where family_ref = 190997

returns

190999
191000
191001
191002

again thats correct

So why would

select i.individual_ref from individual i where not EXISTS (select c.individual_ref from contact c ) 

return no rows ?

I'm expecting

190999
191000
191001

any ideas .

Thanks