Re: [eclipselink-users] Conditional LEFT JOIN in JPQL

[bht@xxxxxxxxxxxxx]

Hi Tom,

Thanks for your helpful reply, and I mean, really helpful and
considerate.

But I think it is fair to say that this stuff is just to simplistic on
the JPQL side and as a consequence of that, JPA overall is too
difficult to use.

Even simple things such as this come at the cost of becoming research
projects with administration overhead of release and bug tracking,
lobbying in specification groups and so on. Where do we find THIS kind
of developmet resources? I think you might agree with me that this is
too much for the average development shop, and I woud appreciate if
Oracle would take the much needed initiative in this area.

Vidas has found a way out of this with native queries as I had found
many months ago, and if he is lucky, then there are no case
sensitivity problems with @SqlResultSetMapping.

But still, one cannot use select * in native queries if multiple
tables are involved as in Vidas's case because of duplicate column
names such as "id", "name" in the tables. Which means that to populate
domain objects one must list every field of every table. It is a
maintenance nightmare, and one gets further and further away from the
original idea of JPA which is to make life easier not more difficult
than with JDBC and DAOs.

I just built hashtables in the application to filter the resultsets as
a workaround for the condition in the outer join because the
workarounds were just too costly. Still messy, lacking performance and
very annoying but at least refactor safe.

JPQL is obviously key to all this, and there hasn't been any
improvement in JPA 2.0, so I am very disappointed, and I want you guys
to be aware of it. JPQL is quite useless except for the most basic
queries, but who writes such basic queries these days?

Thanks again,

Bernard


On Mon, 03 May 2010 16:15:30 -0400, you wrote:

>Hi Bernard,
>
>   Since this is a spec issue, feedback to the spec group is likely the best 
>long term solution.
>
>   In the short to medium term, it is a matter of determining what EclipseLink 
>can do for you.  For our upcoming 2.1 release we have been adding some 
>enhancements to the way queries can be run.  (downcasting and a function 
>operator in JPQL so far and some fetch group support through hints and 
>configuration options), so changes to the way queries work are not out of the 
>question.  We are, however, somewhat limited in the degree to which we can 
>change JPQL - i.e. we have to remain TCK compliant and we do not want to add 
>JPQL that will be disallowed by later specifications. As a result we have been 
>hesitant to address issues in JPQL that require major grammar rewrites.
>
>   Have you looked at the criteria API?  I haven't had a chance to investigate 
>how one would write this kind of query in the criteria API, but it is, in some 
>ways more flexible that JPQL especially with regards to how joins are handled.
>
>   Assuming that is of no help, perhaps your best bet is EclipseLink native API. 
>  Is that an option for you - since you are asking for a non-spec feature anyway?
>
>-Tom
>
>bht@xxxxxxxxxxxxx wrote:
>> Hi Chris,
>> 
>> Thanks for that. It is amazing how many incident records the community
>> had to keep track of with regard to this single issue. I wasn't aware
>> of this one.
>> 
>> You did not mention the JPQL issue. Where is this tracked? As I wrote,
>> I prefer to stay clear of bad practices in standard use cases such as
>> this common outer join application.
>> 
>> This seems to be a bigger one as it is on the specification level,
>> where EL would not have a compliance issue. Is that why you do not
>> mention it?
>> 
>> Many thanks.
>> 
>> Regards
>> 
>> Bernard
>> 
>> On Mon, 03 May 2010 15:13:10 -0400, you wrote:
>> 
>>> Hello Bernard,
>>>
>>> With respect to case and native queries, you will find that EclipseLink 
>>> has worked around the problem of databases returning different cases 
>>> through bug 299926
>>> https://bugs.eclipse.org/bugs/show_bug.cgi?id=299926
>>> If using a recent EclipseLink nightly build, you should not see case 
>>> issues when using native SQL (unless your database is case sensitive). 
>>>
>>> Best Regards,
>>> Chris
>>>
>>> On 03/05/2010 3:03 PM, bht@xxxxxxxxxxxxx wrote:
>>>> Hi Vide,
>>>>
>>>> Re 1)
>>>>
>>>> It should be possible.
>>>>
>>>> However there are major limitations with conditional left join in JPQL
>>>> and this is a BIG problem.
>>>>
>>>> For example, in a product list display, alongside with each list item,
>>>> we would want an indicator showing whether the item exists in the
>>>> user's list of favorite items. An outer join clause with such a
>>>> condition defining the join is the only practical way to do this. This
>>>> is industry standard, except it is NOT supported in JPQL.
>>>>
>>>> In JPQL, You cannot code outer join "ON" something other than what is
>>>> already obvious from relationships. In fact "ON" is not supported.
>>>>
>>>> In SQL you can say:
>>>>
>>>> SELECT d.name, count( v.id ) FROM dealer d LEFT OUTER JOIN vehicle v
>>>> ON v.dealer_id = d.dealer_id AND v.type = 'New' GROUP BY d.name
>>>>
>>>> In JPQL, you only have the WHERE clause which limits your results,
>>>> making the outer join useless:
>>>>
>>>> SELECT d.name, count( v.id ) FROM dealer d LEFT OUTER JOIN
>>>> d.vehicleList v WHERE v.type = 'New' GROUP BY d.name
>>>>
>>>>
>>>> PLease refer to:
>>>>
>>>> http://old.nabble.com/LEFT-OUTER-JOIN---ON-clause-equivalent--td12638986.html#a12641034
>>>>
>>>> One can work around this with a native query, but this is a really
>>>> lame suggestion because there are a number of problems with them -
>>>> they are not best practice. I found them very difficult to work with
>>>> in this scenario because of unexpected case sensitivity issues in the
>>>> query itself (not the data) when using @SqlResultSetMapping. You will
>>>> discover the whole glory when you get there:
>>>>
>>>> http://forums.java.net/jive/thread.jspa?messageID=190092
>>>>     Request for input - TLE case sensitivity
>>>>     Outlines workarounds
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=3698
>>>>     Details workaround
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=600
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=1402
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=1442
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=1507
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=2046
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=887
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=832
>>>> https://glassfish.dev.java.net/issues/show_bug.cgi?id=1027
>>>>
>>>> Unless fields are wrapped in quotes,
>>>>  Oracle returns fields in uppercase,
>>>>  Sybase returns them as they were defined,
>>>>  and Postgres returns them in lowercase.
>>>>
>>>>
>>>>  http://sql-info.de/en/postgresql/postgres-gotchas.html#1_2
>>>>      Unquoted object names fold to lower case (folding behavior)
>>>>
>>>>
>>>>
>>>> Regards
>>>>
>>>> Bernard
>>>>
>>>>
>>>>
>>>> On Mon, 3 May 2010 01:39:31 -0700 (PDT), you wrote:
>>>>
>>>>   
>>>>> Hello,
>>>>>
>>>>>  I have two questions about JOIN's in JPQL:
>>>>> 1. Is it possible to write a conditional LEFT JOIN with JPQL, which in SQL
>>>>> looks like:
>>>>>
>>>>> SELECT a.*, b.*, c.*
>>>>>  FROM entityA a
>>>>>  LEFT JOIN entityB b ON a.id=b.a_id AND b.some_field="some_value"
>>>>>  LEFT JOIN entityC c ON a.id=c.a_id AND c.some_field="some_value"
>>>>>  WHERE some_other_post_conditions
>>>>>
>>>>> 2. Can I use LEFT JOIN's in JPQL on entities, which don't have relationship
>>>>> annotations between each other?
>>>>>
>>>>> Thanks
>>>>>     
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>> 
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