I have 2 tables:

Invoice

CustomerInvoiceID - PK
RecordLocator - Text
createdDate - date

SimplifiedInvoice

CustomerInvoiceID - PK / FK Invoices
CountryCode - text
VatPercentage - number
SubtotalAmount - number
TotalAmount - number

If I run the following query:

select  
    si.CountryCode, 
    si.RecordLocator, 
    si.CustomerInvoiceID, 
    i.CreatedDate,  
    i.VatPercentage, 
    si.SubTotalAmount, 
    si.TotalAmount
from
    SimpliedInvoice si , invoice i 
where 
    si.CustomerInvoiceID = i.CustomerInvoiceID
    and si.CountryCode = 'GR'

My result is an empty dataset.

But if I run this query:

select * 
from SimpliedInvoice
where CustomerInvoiceID in (select CustomerInvoiceID 
                            from Invoice 
                            where CountryCode = 'GR')

I get plenty of data.

I am not able to see why, any idea?