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---+ How do <code>CREATE GRAPH</code>, <code>DROP GRAPH</code>, <code>CLEAR GRAPH</code>, and <code>DELETE FROM <graphname></code> work in Virtuoso?
---++ What?
Here are some details about the <code>CREATE GRAPH</code>, <code>DROP GRAPH</code>, <code>CLEAR GRAPH</code>, and <code>DELETE FROM <graphname></code> commands.
---++ Why?
Clarifying implications of the various commands in Virtuoso.
---++ How?
In Virtuoso, it does not matter whether <code>CREATE GRAPH</code> and <code>DROP GRAPH</code> (or its synonym,
<code>CLEAR GRAPH</code>) are called or not. Their purpose is to provide compatibility with the original SPARUL that
was designed for Jena. Some Jena triple stores require explicit creation of each graph (like <code>CREATE TABLE</code>
in SQL), report errors if one tries to create a graph twice, and so on.
For Virtuoso, a new graph is not an important system event, because Virtuoso has a single quad-store shared by all
graphs. When a graph is made by <code>CREATE GRAPH</code>, its name is placed in an auxiliary table that is used
solely to signal appropriate errors when a user tries to <code>CREATE</code> a graph that has already been
<code>CREATEd</code>, or to <code>DROP</code> a graph which has not been <code>CREATEd</code>; this table is not
used in any way by SPARQL or other subsystems. In a perfect world, smart development tools will query that table
to give hints to a programmer regarding available graphs, but the reality is not so perfect.
There is an important difference between <code>DELETE FROM <graphname> WHERE { ?s ?p ?o }</code> and
<code>CLEAR GRAPH <graphname></code>. Both will delete all triples from the specified graph <code><graphname></code>
with equal speed, but <code>CLEAR GRAPH</code> will also delete free-text index data about occurrences of literals
in this specific graph. <code>CLEAR GRAPH</code> will therefor make the database slightly more compact and the text
search slightly faster. (Naturally, DROP GRAPH makes CLEAR GRAPH inside, not just DELETE FROM...)
---+++ DROP GRAPH and CLEAR GRAPH are equivalent
<code>DROP GRAPH</code> is equivalent of <code>CLEAR GRAPH</code>. Both require that the graph has been explicitly <code>CREATEd</code>.
<i><b>Note:</b> All SPARUL a/k/a SPARQL-Update commands require that the user (i.e., the <code>SPARQL</code> user for the default <code>/sparql</code> endpoint) be granted <b><code><nowiki>SPARQL_UPDATE"</nowiki></code></b> privilege.</i>
Assume the following sequence of commands to be executed from the <code>/sparql</code> endpoint:
1 Explicitly create a graph:
<verbatim>
CREATE GRAPH <qq>
callret-0
Create graph <qq> -- done
</verbatim>
1 If you know the graph was explicitly created, use the simple <code>DROP GRAPH</code>:
<verbatim>
DROP GRAPH <qq>
callret-0
Drop graph <qq> -- done
</verbatim>
1 If you don't know whether a graph was created explicitly or not, you can add the <code>SILENT</code> option:
<verbatim>
DROP SILENT GRAPH <qq>
callret-0
Drop silent graph <qq> -- done
</verbatim>
---++Related
* [[VirtTipsAndTricksGuide][Virtuoso Tips and Tricks Collection]]
* [[http://docs.openlinksw.com/virtuoso/rdfinsertmethods.html][RDF Insert Methods in Virtuoso]]
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