Title: Producing Result Sets

SPARQL has four result forms:

-   SELECT – Return a table of results.
-   CONSTRUCT – Return an RDF graph, based on a template in the
    query.
-   DESCRIBE – Return an RDF graph, based on what the query
    processor is configured to return.
-   ASK – Ask a boolean query.

The SELECT form directly returns a table of solutions as a result
set, while DESCRIBE and CONSTRUCT use the outcome of matching to
build RDF graphs.

## Solution Modifiers

Pattern matching produces a set of solutions. This set can be
modified in various ways:

-   Projection - keep only selected variables
-   OFFSET/LIMIT - chop the number solutions (best used with ORDER
    BY)
-   ORDER BY - sorted results
-   DISTINCT - yield only one row for one combination of variables
    and values.

The solution modifiers OFFSET/LIMIT and ORDER BY always apply to
all result forms. 

### OFFSET and LIMIT

A set of solutions can be abbreviated by specifying the offset (the
start index) and the limit (the number of solutions) to be
returned. Using LIMIT alone can be useful to ensure not too many
solutions are returned, to restrict the effect of some unexpected
situation.  LIMIT and OFFSET can be used in conjunction with
sorting to take a defined slice through the solutions found.

### ORDER BY

SPARQL solutions are sorted by expression, including custom
functions.

    ORDER BY ?x ?y

    ORDER BY DESC(?x)

    ORDER BY x:func(?x)  # Custom sorting condition

### DISTINCT

The SELECT result form can take the DISTINCT modifier which ensures
that no two solutions returned are the same - this takes place
after projection to the requested variables.

 

## SELECT

The `SELECT` result form is a projection, with DISTINCT applied, of
the solution set. `SELECT` identifies which named variables are in
the result set.  This may be "`*`" meaning "all named variables"
(blank nodes in the query act like variables for matching but are
never returned).

## CONSTRUCT

CONSTRUCT builds an RDF based on a graph template.  The graph
template can have variables which are bound by a WHERE clause.  The
effect is to calculate the graph fragment, given the template, for
each solution from the WHERE clause, after taking into account any
solution modifiers. The graph fragments, one per solution, are
merged into a single RDF graph which is the result.

Any blank nodes explicitly mentioned in the graph template are
created afresh for each time the template is used for a solution.

## DESCRIBE

The CONSTRUCT form, takes an application template for the graph
results. The DESCRIBE form also creates a graph but the form of
that graph is provided the query processor, not the application.
For each URI found, or explicitly mentioned in the DESCRIBE clause,
the query processor should provide a useful fragment of RDF, such
as all the known details of a book. ARQ allows domain-specific
description handlers to be written.

## ASK

The ASK result form returns a boolean, true of the pattern matched
otherwise false.

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