Something like that, right?
SELECT COALESCE(b.title, t.name) FROM Order o JOIN TREAT(o.product AS Book) b JOIN TREAT(o.product AS Toaster) t
On 14 mars 2011, at 18:55, Gordon Yorke wrote:
> This could easily be done using the TREAT in the FROM clause.
> --Gordon
>
> Emmanuel Bernard wrote:
>>
>> On 11 mars 2011, at 17:51, Matthew Adams wrote:
>>
>>
>>> On Fri, Mar 11, 2011 at 10:11 AM, Rainer Kwesi Schweigkoffer
>>> <kwesi_at_sap.com> wrote:
>>>
>>>> Matthew Adams, am 10 Mar 2011 hast Du um 17:12 zum Thema "[jsr338-experts] Re: query improvements: downcast" geschrieben :
>>>>
>>>>
>>>>> If so, then I would restate **for the deferred,
>>>>> not-currently-specified projection use of TREAT AS** that if either
>>>>> Book is not assignment-compatible with Product or the filter results
>>>>> in type-incompatible instances with the projection expression, an
>>>>> exception is thrown.
>>>>>
>>>> Do you see a specific use case that would require to admit TREAT AS for
>>>> the select clause as well ?
>>>>
>>>>
>>> I suppose not.
>>>
>>> Theoretically, using TREAT AS in the projection (the select clause) is
>>> not required. If the user wants to exclude certain types from the
>>> query, the mechanism by which I'd recommend that they do it would be
>>> to use FROM ... TREAT ... AS. To use SELECT ... TREAT ... AS requires
>>> that the filter only return objects of the expected type; if they want
>>> objects of that type, then why not just use FROM ... TREAT ... AS? It
>>> seems like they're just asking for an exception to be thrown if they
>>> use SELECT ... TREAT ... AS. The only behavior that they'd get with
>>> SELECT ... TREAT ... AS is either an exception if their query was
>>> wrong or the same behavior as FROM ... TREAT ... AS. Seems
>>> unnecessary to me.
>>>
>>> Let's see if anyone comes up with a use case for SELECT ... TREAT ...
>>> AS that can't be solved with a FROM ... TREAT ... AS. I'd be
>>> surprised if there were.
>>>
>>
>> I can see this use case being acceptable
>>
>> class Item {
>> }
>>
>> class Book extends Item {
>> String getTitle();
>> }
>>
>> class Toaster extends Item {
>> String getName();
>> }
>>
>> SELECT COALESCE( TREAT(o.product AS Book).title, TREAT(o.product AS Toaster).name ) ...