[[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. <>:: 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. <> and <> 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[]