Database Reference
In-Depth Information
TABLE 5.1
SPARQL Algebra and Syntax
Algebra
Syntax
BGP
Set of triple patterns concatenated via.
Join
Concatenation of two groups {…} . {…}
Filter
FILTER
LeftJoin
OPTIONAL
Union
UNION
Graph
GRAPH
"literal"^^type
). If needed in arithmetic expressions, numeric literals (e.g.,
literals of type xsd:integer
) are parsed into the appropriate numeric
data type of Pig Latin at runtime. The RDF syntax does not define an internal struc-
ture of blank nodes, they just have to be distinguishable from URIs and literals.
Thus, we can also represent them as atoms with a leading underscore (
_ :nodeID
).
Hence, an RDF triple can be represented as a tuple with three fields of atomic type
(
chararray
) with schema (
s:chararray, p:chararray, o:chararray
).
5.3.2 a
lgebra
t
ranslation
For each operator of the SPARQL algebra we give a translation into a sequence
of Pig Latin commands illustrated by a representative example (P1-P6). First, we
introduce the needed terminology analogous to [12]: Let
V
be the infinite set of query
variables and
T
be the set of valid RDF terms (URIs, RDF literals, blank nodes).
Definition 5.1
A
(solution) mapping
μ is a partial function μ :
V
→
T
. We call μ(?
v
) the variable bind-
ing of μ for ?
v
. Abusing notation, for a triple pattern
p
we call μ(
p
) the triple pattern that
is obtained by substituting the variables in
p
according to μ. The domain of μ,
dom
(μ),
is the subset of
V
where μ is defined and the domain of
p
,
dom
(
p
), is the subset of
V
used in
p
. The result of a SPARQL query is a multiset of solution mappings Ω.
Definition 5.2
Two mappings μ
1
, μ
2
are compatible iff, for every variable ?
v
∈
dom
(μ
1
) ∩
dom
(μ
2
), it
holds that μ
1
(?
v
) = μ
2
(?
v
). It follows that mappings with disjoint domains are always
compatible and the set-union (merge) of μ
1
and μ
2
, μ
1
∪ μ
2
, is also a mapping.
Basic Graph Pattern (BGP).
BGPs are the basis of all SPARQL queries as it is
the only operator that is evaluated directly on the underlying RDF data. The result
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