[[java-joining-queries]]
=== Joining queries

Performing full SQL-style joins in a distributed system like Elasticsearch is
prohibitively expensive.  Instead, Elasticsearch offers two forms of join
which are designed to scale horizontally.

<<java-query-dsl-nested-query,`nested` query>>::

Documents may contains fields of type `nested`. These
fields are used to index arrays of objects, where each object can be queried
(with the `nested` query) as an independent document.

<<java-query-dsl-has-child-query,`has_child`>> and <<java-query-dsl-has-parent-query,`has_parent`>> queries::

A parent-child relationship can exist between two
document types within a single index. The `has_child` query returns parent
documents whose child documents match the specified query, while the
`has_parent` query returns child documents whose parent document matches the
specified query.

include::nested-query.asciidoc[]

include::has-child-query.asciidoc[]

include::has-parent-query.asciidoc[]


