Question

The following (simplified and obfuscated) query appears to (in my
mind) return a varchar type from each possible return branch in the
nested case expression. I went to the trouble of casting the first
two possible return branches to varchar to ensure their congruence
with the last two possible return types as represented in the last
(most deeply nested) case statement which returns strings. However,
when run, it complains about trying to convert the string ‘foo’ to an
int. Everything worked fine until I added that bottommost, most
deeply nested, case statement that always returs strings (and indeed,
before I added that, there was no need for any conversion of dates
since dates were all that could ever be returned). QUESTION: Any
idea why sql server is trying to convert the ‘foo’ string (and
probably ‘bar …’ string if it could get that far) into an int? More
importantly, any ideas on a solution to solve the casting/conversion
issue? Thanks in advance.

select case
         when t1.flubbitz - t1.dubbitz <= 0
         then 
           case
             when (select min(t3.[duedate])
                   from blatz t3  
                     inner join dort t4 on t3.anumerickey = t4.anumerickey 
                   where t4.[somenumbercol] = t2.somenumbercol
                     and t4.somecode = '01'
                       and t3.duedate > getdate()) is not null
             then (select left(convert(varchar, (select min(t3.[duedate])
                   from blatz t3  
                     inner join dort t4 on t3.anumerickey = t4.anumerickey 
                   where t4.[somenumbercol] = t2.somenumbercol
                     and t4.somecode = '01'
                       and t3.duedate > getdate()), 101), 10))
             else 
               case 
                 when (select min(t0.[duedate])
                       from sabtz t0  
                             inner join quux t1 on t0.anumerickey = t1.anumerickey 
                       where t0.somenumbercol = t2.somenumbercol
                         and t0.duedate > getdate()) is not null
                 then (select left(convert(varchar, (select min(t0.[duedate])
                       from sabtz t0  
                             inner join quux t1 on t0.anumerickey = t1.anumerickey 
                       where t0.somenumbercol = t2.somenumbercol
                         and t0.duedate > getdate()), 101), 10))
                     else 
                   case
                     when t2.somenumbercol like 'blub%' then 'foo'
                     when t2.somenumbercol like 'batz%' then 'bar ' + left(convert(varchar, getdate(), 101), 10) 
                   end    
               end 
           end       
         else (select null) 
       end as [Foobar]

from yyy t1 
     inner join xxx t2 on t1.somenumbercol = t2.somenumbercol

where ...

Answer 1

It’s the “(SELECT NULL)”.

In the examples below, “(SELECT NULL)” throws the error and “NULL” does not. The “()” casts the NULL as an INT.

SELECT CASE -- error
WHEN 3 < 2 THEN 'Hello'
ELSE (SELECT NULL)
END
UNION
SELECT 'Goodbye'

SELECT CASE -- works
WHEN 3 < 2 THEN 'Hello'
ELSE NULL
END
UNION
SELECT 'Goodbye'

SELECT CASE -- works
WHEN 3 < 2 THEN 'Hello'
ELSE CAST(NULL AS VARCHAR)
END
UNION
SELECT 'Goodbye'