Case within case in join

Question

I am trying to set up a join where what fields are matched depend on the value of another field but cannot seem to get the syntax right. I have only posted part of the query as it is quite long and the rest has no relevance to the join.
The error is
Msg 102, Level 15, State 1, Line 59
Incorrect syntax near '='.
on the line
then  CertNo =
Can anybody help me sort this out please?
regards
Ron
from
InvoiceItems
join Invoices on inv_no = invit_invno and inv_canind <> 'Y' and inv_cust <> '$sayrf'
join Customers on cust_no = inv_cust
join Stock on stock_no = invit_partno
join Batches on batch_batch = invit_batch and batch_part = invit_partno
inner join Certtable on
case when certpord = ''
then CertNo =
CASE WHEN LEFT(invit_cert,5) = 'HAPAG' then 'HAPAGL'
WHEN LEFT(invit_cert,5) = 'UAMMA' then 'UAMMAERSK'
WHEN LEFT(invit_cert,5) = 'UAMRB' then 'UAMRB211'
WHEN LEFT(invit_cert,5) = 'NSKCON' then 'NSKCON'
ELSE invit_cert
end
else certpord = batch_order
end
join Suppliers on supplierRef = supp_no

Answer 1

Because you are thinking of CASE as a control-of-flow construct - it is not.  It is an expression that returns a scalar value.  You can use it within a join clause but it must be done correctly.  Case returns a scalar value which must be used
to form a Boolean expression.  You currently have
case when certpord = ''
    then  CertNo =
    CASE WHEN LEFT(invit_cert,5) = 'HAPAG' then 'HAPAGL'
    WHEN LEFT(invit_cert,5) = 'UAMMA' then 'UAMMAERSK'
    WHEN LEFT(invit_cert,5) = 'UAMRB' then 'UAMRB211'
    WHEN LEFT(invit_cert,5) = 'NSKCON' then 'NSKCON'
    ELSE invit_cert
    end
    else certpord = batch_order
    end
Reducing this to remove the specific logic within the nested expression, you have
case when certpord = ''
    then  CertNo =  [c1]
    else certpord = batch_order
    end
So it should be obvious that you are attempting to treat the case expression as a control-of-flow statement.  Usually it is easier to write the logic twice and then combine the queries into a single one (if that is possible).  I'm guessing that
you need something like:
inner join Certtable on
(certprod = '' and (case left(invit_cert,5) when 'HAPAG' then 'HAPAGL' ... else invit_cert end) = CertNo)
or (certpord <> '' and certpord = batch_order)
Carefully consider the logic if certprod can be null.  And a last comment. You should get into the many best practices habits - one of which is to give an alias to each table and to use always the alias when referencing columns in each table. 
This makes it easier to find logic problems based on the presence of similar column names in different tables and it improves the ability of your reader to understand the query and the association of columns to tables and tables to tables.  If you
find that the logic used to translate left(invit_cert, 5) appears frequently, you should consider adding a computed column to the associated table (and perhaps materializing it and /or the relationship). 
And another note - don't write/implement code that serves no useful purpose.  The last condition of your embedded case expression does nothing useful. 
WHEN LEFT(invit_cert,5) = 'NSKCON' then 'NSKCON'
The above condition can be safely left out of the logic since it will be evaluated in the same fashion as the else condition of the case expression.