[jsr338-experts] Re: query improvements: downcasting
Yes.
--Gordon
Emmanuel Bernard wrote:
> Something like that, right?
>
> SELECT COALESCE(b.title, t.name <http://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 ) ...
>