teradata SQL tuning pundits - SELECT Failed. 3771: Illegal expression in WHEN clause of CASE expression

Question

I am using a statement as below and get this error:

SELECT Failed. 3771: Illegal expression in WHEN clause of CASE expression.

I had better hopes from Teradata. SQL Server can do it but Teradata can't. How can I work around this? Any solution?

sel ( CASE
    WHEN  EXISTS   ( sel '1' from VolatileTable Dtb1 where Dtb1.c1=FACT_Table_5MillionRows.C1)
    THEN "FACTTablew5MillionRows"."CustomColumName" 
ELSE 'ALL OTHER'
END  ) (NAMED "CustomColumName" )

from
"Db"."FACTTablew5MillionRows" 

Answer 1

Teradata doesn't like EXISTS in Correlated Scalar Subqueries within a CASE, but you can rewrite it like this:

select
  ( CASE
       WHEN C1 = ( select MIN(C1) from VolatileTable Dtb1 
                   where Dtb1.c1=ft.C1)
       THEN ft."CustomColumName" 
       ELSE 'ALL OTHER'
    END  ) (NAMED "CustomColumName" )
from
"Db"."FACTTablew5MillionRows" as ft

If VolatileTable.C1 is unique you can remove the MIN.

But in 95% logic like this can be replaced by a LEFT JOIN:

select
   ft.*,
   CASE WHEN Dtb1.c1 IS NOT NULL 
        THEN ft."CustomColumName" 
        ELSE  'ALL OTHER' 
   end as "CustomColumName" 
from "Db"."FACTTablew5MillionRows" as ft
left join VolatileTable Dtb1 
on Dtb1.c1=ft.C1

This will return duplicated rows if VolatileTable.C1 is not unique, then you need to change it to:

from "Db"."FACTTablew5MillionRows" as ft
left join (select distinct C1 from VolatileTable) Dtb1 
on Dtb1.c1=ft.C1