15.15. Implicit joins (path expressions)

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Another means of adding to the scope of object model types available to the query is through the use of implicit joins or path expressions.

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<div id="hql-implicit-join-example" class="exampleblock">
<div class="title">Example 374. Simple implicit join example</div>
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<pre class="prettyprint highlight">`List&lt;Phone&gt; phones = entityManager.createQuery(
    "select ph " +
    "from Phone ph " +
    "where ph.person.address = :address ", Phone.class )
.setParameter( "address", address )
.getResultList();

// same as
List&lt;Phone&gt; phones = entityManager.createQuery(
    "select ph " +
    "from Phone ph " +
    "join ph.person pr " +
    "where pr.address = :address ", Phone.class )
.setParameter( "address", address)
.getResultList();`</pre>
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An implicit join always starts from an `identification variable`, followed by the navigation operator ( `.` ),
followed by an attribute for the object model type referenced by the initial `identification variable`.
In the example, the initial `identification variable` is `ph` which refers to the `Phone` entity.
The `ph.person` reference then refers to the `person` attribute of the `Phone` entity.
`person` is an association type so we further navigate to its age attribute.

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If the attribute represents an entity association (non-collection) or a component/embedded, that reference can be further navigated.
Basic values and collection-valued associations cannot be further navigated.

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As shown in the example, implicit joins can appear outside the `FROM clause`.
However, they affect the `FROM clause`.

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Implicit joins are always treated as inner joins.

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Multiple references to the same implicit join always refer to the same logical and physical (SQL) join.

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<div id="hql-implicit-join-alias-example" class="exampleblock">
<div class="title">Example 375. Reused implicit join</div>
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<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight">`List&lt;Phone&gt; phones = entityManager.createQuery(
    "select ph " +
    "from Phone ph " +
    "where ph.person.address = :address " +
    "  and ph.person.createdOn &gt; :timestamp", Phone.class )
.setParameter( "address", address )
.setParameter( "timestamp", timestamp )
.getResultList();

//same as
List&lt;Phone&gt; phones = entityManager.createQuery(
    "select ph " +
    "from Phone ph " +
    "inner join ph.person pr " +
    "where pr.address = :address " +
    "  and pr.createdOn &gt; :timestamp", Phone.class )
.setParameter( "address", address )
.setParameter( "timestamp", timestamp )
.getResultList();`</pre>
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<div class="paragraph">

Just as with explicit joins, implicit joins may reference association or component/embedded attributes.
For further information about collection-valued association references, see [Collection member references](#hql-collection-valued-associations).

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In the case of component/embedded attributes, the join is simply logical and does not correlate to a physical (SQL) join.
Unlike explicit joins, however, implicit joins may also reference basic state fields as long as the path expression ends there.

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