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             <h1 class="title">SPARQL Tutorial - Alternatives in a Pattern</h1>

 	<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>


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	<li class="active">SPARQL UNION</li>




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	</div>
	<h1 class="title">SPARQL Tutorial - Alternatives in a Pattern</h1>

	<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’s names. In vCard, it is vCard:FN, the “formatted
	name”, 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: <http://xmlns.com/foaf/0.1/> .
	@prefix vcard: <http://www.w3.org/2001/vcard-rdf/3.0#> .

	_:a foaf:name "Matt Jones" .

	_:b foaf:name "Sarah Jones" .

	_:c vcard:FN "Becky Smith" .

	_:d vcard:FN "John Smith" .
	</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: <http://xmlns.com/foaf/0.1/>
	PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#>

	SELECT ?name
	WHERE
	{
	{ [] foaf:name ?name } UNION { [] vCard:FN ?name }
	}
	</code></pre>
	<p>This returns the results:</p>
	<pre><code>-----------------
	\| name \|
	=================
	\| "Matt Jones" \|
	\| "Sarah Jones" \|
	\| "Becky Smith" \|
	\| "John Smith" \|
	-----------------
	</code></pre>
	<p>It didn’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: <http://xmlns.com/foaf/0.1/>
	PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#>

	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: <http://xmlns.com/foaf/0.1/>
	PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#>

	SELECT ?name1 ?name2
	WHERE
	{
	{ [] foaf:name ?name1 } UNION { [] vCard:FN ?name2 }
	}

	---------------------------------
	\| name1 \| name2 \|
	=================================
	\| "Matt Jones" \| \|
	\| "Sarah Jones" \| \|
	\| \| "Becky Smith" \|
	\| \| "John Smith" \|
	---------------------------------
	</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’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: <http://xmlns.com/foaf/0.1/>
	PREFIX vCard: <http://www.w3.org/2001/vcard-rdf/3.0#>

	SELECT ?name1 ?name2
	WHERE
	{
	?x a foaf:Person
	OPTIONAL { ?x foaf:name ?name1 }
	OPTIONAL { ?x vCard:FN ?name2 }
	}

	---------------------------------
	\| name1 \| name2 \|
	=================================
	\| "Matt Jones" \| \|
	\| "Sarah Jones" \| \|
	\| \| "Becky Smith" \|
	\| \| "John Smith" \|
	---------------------------------
	</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>


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