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

  Thanks for the lesson.
With your help and Bernards', I have managed to get this feature on an initial list for consideration for the next JPA specification. The spec is still in the very early stages, and I do not have any idea where it fits in terms of priority compared to the other items being considered.
If you are interested in an EclispeLink-native-api solution, I can do some experimentation to see what EclipseLink can provide. The basic idea is we would define something called a OneToOneQueryKey using the fields you want to join ON and then query across that query key. 

Let me know if this kind of solution is of interest to you,
Tom

Vidas wrote:

Hi, Tom,

2010/5/5 Tom Ware <tom.ware@xxxxxxxxxx <mailto:tom.ware@xxxxxxxxxx>>

    I appologize for my lack of understanding but...

    How will the results of the following queries be different (assuming
    a mapped relationship between dealer and vehicle using v.dealer_id =
    d.dealer_id  as the foreign key relationship)?


    1. SELECT d.name <http://d.name>, count(v.id <http://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
    <http://d.name>

    2. SELECT d.name <http://d.name>, count(v.id <http://v.id>) FROM
    dealer d LEFT OUTER JOIN d.vehicles v where v.type = 'New' or v.type
    isnull GROUP BY d.name <http://d.name>


Second query will be missing dealers, who only have old vehicles.

Taking other Bernards example:
1. SELECT p.name <http://p.name/>, f.id <http://f.id/> FROM product p LEFT OUTER JOIN product_favorite f 
ON (p.id <http://p.id/> = f.product_id AND f.user_id = :userId)
2. SELECT p.name <http://p.name/>, f.id <http://f.id/> FROM product p LEFT OUTER JOIN p.product_favorite f 
WHERE f.user_id = :userId OR f.user_id isnull
Second query will be missing products, which are favorites only to other users. 

And my case:
SELECT c.name <http://c.name/>, e1.fieldValue AS language, e2.fieldValue AS nationality 
  FROM Country c
LEFT JOIN ExtCharFields e1 ON c.id <http://c.id/>=e1.country_id AND e1.fieldName = "language" LEFT JOIN ExtCharFields e2 ON c.id <http://c.id/>=e2.country_id AND e2.fieldName = "nationality" 

It isn't possible to write JPQL query at all


--
Sincerely,
Vidas


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