# SPARQL as understood by Tracker

This section describes the choices made by Tracker in its interpretation of the SPARQL documents, as well as its extensions and divergences.

## The default graph

The SPARQL documentation says:

    Operations may specify graphs to be modified, or they may rely on a
    default graph for that operation. […] The unnamed graph for the store
    will be the default graph for any operations on that store. Depending
    on implementation, the unnamed graph may refer to a separate graph, a
    graph describing the named graphs, a representation of a union of
    other graphs, etc.

Tracker defines the default graph to be the union of the unnamed graph and all known named graphs. Updates without specified graph are still performed only on the unnamed graph.

## Blank nodes

The RDF documentation says:

    A blank node is a node that is not a URI reference or a literal. In
    the RDF abstract syntax, a blank node is just a unique node that can
    be used in one or more RDF statements, but has no intrinsic name.

By default Tracker treats blank nodes as an URI generator instead. The string referencing a blank node (e.g. as returned by cursors) permanently identifies that blank node and can be used as an URI reference in future queries.

The blank node behavior defined in the RDF/SPARQL specifications can be enabled with the TRACKER_SPARQL_CONNECTION_FLAGS_ANONYMOUS_BNODES flag.

## Property functions

The SPARQL documentation says:

    In addition, SPARQL provides the ability to invoke arbitrary functions
    […]. These functions are invoked by name (an IRI) within a SPARQL query.

Tracker allows using all defined `rdf:Property` instances as functions. If the property has multiple values, it will propagate to the cursor as the `GROUP_CONCAT` (with comma separator) of the multiple values.

## Syntax extensions

Tracker offers some SPARQL syntax extensions. These predate the existence of SPARQL 1.1 and stay for legacy reasons. These extensions should be used sparingly, if at all.

### GROUP_CONCAT

The SPARQL specifications define the following syntax to use a specific separator for the GROUP_CONCAT operation:

    GROUP_CONCAT (?var, separator=;)

Tracker additionally accepts a simplified syntax:

    GROUP_CONCAT (?u, ';')

### BOUND

The BOUND function, as defined in the SPARQL specification, only accepts variables as its single argument. Tracker additionally allows this function to deal with expressions, mainly allowing the nesting of other functions, e.g. functional properties:

    SELECT BOUND (nfo:fileName (?u)) { ?u a nfo:FileDataObject }

### Subselects in expressions

Tracker accepts subselects in place of expressions, these subselects should return a single variable in order to act as a expression. E.g. this query:

    SELECT (SELECT ?ret { ?u nie:hasPart ?elem }) { ?elem a nfo:Folder }

Would be equivalent to this:

    SELECT nie:hasPart(?elem) { ?elem a nfo:Folder }

### INSERT/DELETE SILENT

Tracker allows the use of SILENT after INSERT and DELETE keywords. Errors will be consequently silenced.

### INSERT OR REPLACE

Tracker adds a special `INSERT OR REPLACE` operation. This form of update will overwrite any existing values.

    INSERT OR REPLACE { <file:///> nfo:fileName 'root' }

This operation works the same independently of the cardinality, multi-valued properties are cleared before the insertion.

If clearing a property is desired within the operation, the value list may also contain the NULL keyword, e.g.:

    INSERT OR REPLACE { <file:///> nie:hasPart <a>, <b>, NULL, <c> }

Note that the `DELETE { … } INSERT { … } WHERE { … }` syntax available in SPARQL 1.1 is a more versatile replacement.

### Expressions in ORDER BY

Tracker allows the use of expressions in `ORDER BY` clauses, e.g.:

    SELECT {
      # …
    }
    ORDER BY (?a - ?b)

### Variable names in SELECT clauses

The SPARQL 1.1 specifications enforce that all expressions returned in a `SELECT` clause are set a variable name, e.g.:

    SELECT ((?a - ?b) AS ?sub) {
      # …
    }

Tracker relaxes this restriction, and does not enforce that expressions are surrounded by parentheses, e.g.:

    SELECT ?a + ?b ?a - ?b AS ?sub {
      # …
    }

Note that this hinders query readability (e.g. the example above returns 2 values, an unnamed sum expression, and a named subtraction), so its use is not recommended.

### Separator of update queries

Tracker makes the use of the `;` separator between update clauses optional. Its use is still recommended for readability.

### CONSTRAINT syntax

Tracker supports `CONSTRAINT GRAPH` and `CONSTRAINT SERVICE` clauses in the query prologue. These clauses limit the access outside of the specified graphs and services.

    # Only triples in the tracker:Audio graph will be returned
    CONSTRAINT GRAPH tracker:Audio
    SELECT * { ?s ?p ?o }

If a graph is specified within the query, but not allowed by a `CONSTRAINT GRAPH` clause, it will be effectively interpreted as an empty graph.

If a service is accessed within the query, but not allowed by a `CONSTRAINT SERVICE` clause, it will be interpreted as an error, unless `SERVICE SILENT` syntax is used. In that case it will be interpreted as an empty graph.

The `CONSTRAINT` clauses cannot be contradicted, multiple `CONSTRAINT` clauses effectively intersect the set of allowed graphs/services with previous clauses.

    CONSTRAINT GRAPH tracker:Video, tracker:Audio
    CONSTRAINT GRAPH tracker:Video
    # Only tracker:Video graph can be accessed
    SELECT * { ?s ?p ?o }

    CONSTRAINT GRAPH tracker:Video
    CONSTRAINT GRAPH tracker:Video, tracker:Audio
    # Only tracker:Video graph can be accessed
    SELECT * { ?s ?p ?o }

Disjoint sets result in an empty set of accessible graphs and services.

    CONSTRAINT GRAPH tracker:Video
    CONSTRAINT GRAPH tracker:Audio
    # There are no accessible graphs, this query returns no results
    SELECT * { ?s ?p ?o }

## Mapping IDs and IRIs

Tracker provides the `tracker:id` and `tracker:uri` SPARQL functions, that allow converting an URI reference to a numeric identifier, and back.

These identifiers are expected to be valid a long as the URI is referenced in the store. The existence of these SPARQL functions mostly obey legacy reasons and its use is not recommended.

## Parameters and prepared statements

Tracker accepts `~` prefixed variables in place of literals throughout most of the SPARQL select syntax. These variables are treated as parameters at query time, so it is possible to prepare a query statement once and reuse it many times assigning different values to those parameters at query time.

See TrackerSparqlStatement documentation for more information.

## Full-text search

Tracker provides full-text search capabilities, these are exposed as a `fts:match` pseudo-property that will match the resources matching the given text string.

To complement this pseudo property, Tracker provides the `fts:snippet`, `fts:offsets` and `fts:rank` SPARQL functions that can be used on the matches.

The ontology needs to define the properties that are matched via this full-text search mechanism, by toggling the `tracker:fulltextIndexed` property on in the text `rdf:Property` instances. See the documentation on defining ontologies.

## DESCRIBE queries

The SPARQL documentation says:

    The DESCRIBE form returns a single result RDF graph containing RDF data about resources.

In order to allow serialization to RDF formats that allow expressing graph information (e.g. Trig), DESCRIBE resultsets have 4 columns for subject / predicate / object / graph information.

The results of the search are