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<li class="breadcrumb-item active">SPARQL UNION</li>
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<h1 class="title">SPARQL Tutorial - Alternatives in a Pattern</h1>
<main class="d-flex flex-xl-row flex-column">
<aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block">
<h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2>
<nav id="TableOfContents">
<ul>
<li><a href="#union---two-ways-to-the-same-data">UNION - two ways to the same data</a></li>
<li><a href="#union---remembering-where-the-data-was-found">UNION - remembering where the data was found.</a></li>
<li><a href="#optional-and-union">OPTIONAL and UNION</a></li>
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<article class="flex-column me-lg-4">
<p>Another way of dealing with the semi-structured data is to query
for one of a number of possibilities. This section covers <code>UNION</code>
patterns, where one of a number of possibilities is tried.</p>
<h2 id="union---two-ways-to-the-same-data">UNION - two ways to the same data</h2>
<p>Both the vCard vocabulary and the FOAF vocabulary have properties
for people&rsquo;s names.  In vCard, it is vCard:FN, the &ldquo;formatted
name&rdquo;, and in FOAF, it is foaf:name. In this section, we will look
at a small set of data where the names of people can be given by
either the FOAF or the vCard vocabulary.</p>
<p>Suppose we have <a href="sparql_data/vc-db-3.ttl">an RDF graph</a> that contains name
information using both the vCard and FOAF vocabularies.</p>
<pre><code>@prefix foaf: &lt;http://xmlns.com/foaf/0.1/&gt; .
@prefix vcard: &lt;http://www.w3.org/2001/vcard-rdf/3.0#&gt; .
_:a foaf:name &quot;Matt Jones&quot; .
_:b foaf:name &quot;Sarah Jones&quot; .
_:c vcard:FN &quot;Becky Smith&quot; .
_:d vcard:FN &quot;John Smith&quot; .
</code></pre>
<p>A query to access the name information, when it can be in either
form, could be (<a href="sparql_data/q-union1.rq">q-union1.rq</a>):</p>
<pre><code>PREFIX foaf: &lt;http://xmlns.com/foaf/0.1/&gt;
PREFIX vCard: &lt;http://www.w3.org/2001/vcard-rdf/3.0#&gt;
SELECT ?name
WHERE
{
{ [] foaf:name ?name } UNION { [] vCard:FN ?name }
}
</code></pre>
<p>This returns the results:</p>
<pre><code>-----------------
| name |
=================
| &quot;Matt Jones&quot; |
| &quot;Sarah Jones&quot; |
| &quot;Becky Smith&quot; |
| &quot;John Smith&quot; |
-----------------
</code></pre>
<p>It didn&rsquo;t matter which form of expression was used for the name,
the ?name variable is set. This can be achieved using a <code>FILTER</code> as
this query (<a href="sparql_data/q-union1alt.rq">q-union-1alt.rq</a>) shows:</p>
<pre><code>PREFIX foaf: &lt;http://xmlns.com/foaf/0.1/&gt;
PREFIX vCard: &lt;http://www.w3.org/2001/vcard-rdf/3.0#&gt;
SELECT ?name
WHERE
{
[] ?p ?name
FILTER ( ?p = foaf:name || ?p = vCard:FN )
}
</code></pre>
<p>testing whether the property is one URI or another. The solutions
may not come out in the same order.  The first form is more likely
to be faster, depending on the data and the storage used, because
the second form may have to get all the triples from the graph to
match the triple pattern with unbound variables (or blank nodes) in
each slot, then test each <code>?p</code> to see if it matches one of the
values. It will depend on the sophistication of the query optimizer
as to whether it spots that it can perform the query more
efficiently and is able to pass the constraint down as well as to
the storage layer.</p>
<h2 id="union---remembering-where-the-data-was-found">UNION - remembering where the data was found.</h2>
<p>The example above used the same variable in each branch. If
different variables are used, the application can discover which
sub-pattern caused the match (<a href="sparql_data/q-union2.rq">q-union2.rq</a>):</p>
<pre><code>PREFIX foaf: &lt;http://xmlns.com/foaf/0.1/&gt;
PREFIX vCard: &lt;http://www.w3.org/2001/vcard-rdf/3.0#&gt;
SELECT ?name1 ?name2
WHERE
{
{ [] foaf:name ?name1 } UNION { [] vCard:FN ?name2 }
}
---------------------------------
| name1 | name2 |
=================================
| &quot;Matt Jones&quot; | |
| &quot;Sarah Jones&quot; | |
| | &quot;Becky Smith&quot; |
| | &quot;John Smith&quot; |
---------------------------------
</code></pre>
<p>This second query has retained information of where the name of the
person came from by assigning the name to different variables.</p>
<h2 id="optional-and-union">OPTIONAL and UNION</h2>
<p>In practice, <code>OPTIONAL</code> is more common than <code>UNION</code> but they both
have their uses. <code>OPTIONAL</code> are useful for augmenting the solutions
found, <code>UNION</code> is useful for concatenating the solutions from two
possibilities. They don&rsquo;t necessary return the information in the
same way:</p>
<p>Query(<a href="sparql_data/q-union3.rq">q-union3.rq</a>):</p>
<pre><code>PREFIX foaf: &lt;http://xmlns.com/foaf/0.1/&gt;
PREFIX vCard: &lt;http://www.w3.org/2001/vcard-rdf/3.0#&gt;
SELECT ?name1 ?name2
WHERE
{
?x a foaf:Person
OPTIONAL { ?x foaf:name ?name1 }
OPTIONAL { ?x vCard:FN ?name2 }
}
---------------------------------
| name1 | name2 |
=================================
| &quot;Matt Jones&quot; | |
| &quot;Sarah Jones&quot; | |
| | &quot;Becky Smith&quot; |
| | &quot;John Smith&quot; |
---------------------------------
</code></pre>
<p>but beware of using <code>?name</code> in each <code>OPTIONAL</code> because that is an
order-dependent query.</p>
<p><a href="sparql_datasets.html">Next: Named Graphs</a></p>
</article>
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<h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2>
<nav id="TableOfContents">
<ul>
<li><a href="#union---two-ways-to-the-same-data">UNION - two ways to the same data</a></li>
<li><a href="#union---remembering-where-the-data-was-found">UNION - remembering where the data was found.</a></li>
<li><a href="#optional-and-union">OPTIONAL and UNION</a></li>
</ul>
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