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https://issues.apache.org/jira/browse/JDO-758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15715717#comment-15715717
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Craig L Russell commented on JDO-758:
-------------------------------------

We also need to make sure this section and 14.6.13 Single-string Query element 
binding are consistent.

> What can a subquery return?
> ---------------------------
>
>                 Key: JDO-758
>                 URL: https://issues.apache.org/jira/browse/JDO-758
>             Project: JDO
>          Issue Type: Improvement
>          Components: specification
>    Affects Versions: JDO 3.1
>            Reporter: Tilmann Zäschke
>            Priority: Minor
>             Fix For: JDO 3.2
>
>
> The current spec specifies that subqueries should support ordering and 
> grouping, suggesting that subqueries should be able to return collections of 
> results.
> However, a  Java {{Collection}} (others are not supported in JDOQL) only 
> support {{size()}}, {{isEmpty()}} and {{contains()}}. These three functions 
> all return scalars, furthermore, these could all be implemented easily with 
> an additional term inside the subquery. Since the current reference 
> implementation does not support all these functions, it should be considered 
> whether subqueries should at all be required to return collections of objects.
> A similar argument could be made for returning persistent objects from 
> subqueries (suggested by required support for 'unique'). Persistent objects 
> cannot be used in the parent query, except for accessing their identity, or 
> possibly casting them to a persistent type and access a field.
> Can anyone think of a usecase or other argument why subqueries should be able 
> to return anything else than scalars/boolean (and possibly single persistent 
> objects)?



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