15.13. Root entity references

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A root entity reference, or what JPA calls a `range variable declaration`, is specifically a reference to a mapped entity type from the application.
It cannot name component/ embeddable types.
And associations, including collections, are handled in a different manner, as later discussed.

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The BNF for a root entity reference is:

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<pre class="prettyprint highlight">`root_entity_reference ::=
    entity_name [AS] identification_variable`</pre>
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<div class="title">Example 366. Simple query example</div>
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<pre class="prettyprint highlight">`List&lt;Person&gt; persons = entityManager.createQuery(
    "select p " +
    "from org.hibernate.userguide.model.Person p", Person.class )
.getResultList();`</pre>
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We see that the query is defining a root entity reference to the `org.hibernate.userguide.model.Person` object model type.
Additionally, it declares an alias of `p` to that `org.hibernate.userguide.model.Person` reference, which is the identification variable.

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Usually, the root entity reference represents just the `entity name` rather than the entity class FQN (fully-qualified name).
By default, the entity name is the unqualified entity class name, here `Person`

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<div class="title">Example 367. Simple query using entity name for root entity reference</div>
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<pre class="prettyprint highlight">`List&lt;Person&gt; persons = entityManager.createQuery(
    "select p " +
    "from Person p", Person.class )
.getResultList();`</pre>
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Multiple root entity references can also be specified, even when naming the same entity.

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<div id="hql-multiple-root-reference-jpql-example" class="exampleblock">
<div class="title">Example 368. Simple query using multiple root entity references</div>
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<pre class="prettyprint highlight">`List&lt;Object[]&gt; persons = entityManager.createQuery(
    "select distinct pr, ph " +
    "from Person pr, Phone ph " +
    "where ph.person = pr and ph is not null", Object[].class)
.getResultList();`</pre>
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<pre class="prettyprint highlight">`List&lt;Person&gt; persons = entityManager.createQuery(
    "select distinct pr1 " +
    "from Person pr1, Person pr2 " +
    "where pr1.id &lt;&gt; pr2.id " +
    "  and pr1.address = pr2.address " +
    "  and pr1.createdOn &lt; pr2.createdOn", Person.class )
.getResultList();`</pre>
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