7.3. Associations and joins

You may also assign aliases to associated entities, or even to elements of a collection of values, using a join.

select cat, mate, kitten from Cat as cat 
    inner join cat.mate as mate
    left outer join cat.kittens as kitten

select cat, mate, kitten from Cat as cat 
    inner join cat.mate as mate
    left outer join cat.kittens as kitten

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select cat from Cat as cat left join cat.mate.kittens as kittens

select cat from Cat as cat left join cat.mate.kittens as kittens

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The supported join types are borrowed from ANSI SQL

inner join
left outer join

The inner join, left outer join constructs may be abbreviated.

select cat, mate, kitten from Cat as cat 
    join cat.mate as mate
    left join cat.kittens as kitten

select cat, mate, kitten from Cat as cat 
    join cat.mate as mate
    left join cat.kittens as kitten

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In addition, a "fetch" join allows associations or collections of values to be initialized along with their parent objects, using a single select. This is particularly useful in the case of a collection. It effectively overrides the fetching options in the associations and collection mapping metadata. See the Performance chapter of the Hibernate reference guide for more information.

select cat from Cat as cat 
    inner join fetch cat.mate
    left join fetch cat.kittens

select cat from Cat as cat 
    inner join fetch cat.mate
    left join fetch cat.kittens

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A fetch join does not usually need to assign an alias, because the associated objects should not be used in the where clause (or any other clause). Also, the associated objects are not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason we might need an alias is if we are recursively join fetching a further collection:

select cat from Cat as cat 
    inner join fetch cat.mate
    left join fetch cat.kittens child
    left join fetch child.kittens

select cat from Cat as cat 
    inner join fetch cat.mate
    left join fetch cat.kittens child
    left join fetch child.kittens

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Note that the fetch construct may not be used in queries called using scroll() or iterate(). Nor should fetch be used together with setMaxResults() or setFirstResult(). It is possible to create a cartesian product by join fetching more than one collection in a query (as in the example above), be careful the result of this product isn't bigger than you expect. Join fetching multiple collection roles also sometimes gives unexpected results for bag mappings, so be careful about how you formulate your queries in this case.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties immediately (in the first query) using fetch all properties. This is Hibernate specific option:

select doc from Document doc fetch all properties order by doc.name

select doc from Document doc fetch all properties order by doc.name

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select doc from Document doc fetch all properties where lower(doc.name) like '%cats%'

select doc from Document doc fetch all properties where lower(doc.name) like '%cats%'

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7.3. Associations and joins

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