This section defines mappings from SQL data values to RDF literals:
The following mappings from SQL data values are defined:
TIMESTAMP is used in an IRI
template. The natural
RDF lexical form
corresponding to a SQL data value is the
lexical form of its corresponding
natural RDF literal,
with the additional constraint that the
canonical lexical representation
[XMLSCHEMA2]
SHOULD be chosen.rr:datatype.The mappings cover all predefined SQL 2008 datatypes except
INTERVAL.
INTERVAL, vendor-specific types and types added to future SQL specifications should be mapped to RDF with extensions to the mapping to natural rdf literals. See below.
These mappings are referenced in the R2RML term generation rules.
An informative summary of XSD lexical forms is provided in this specification to aid implementers.
The natural RDF literal corresponding to a SQL data value is the result of applying the following steps:
1, +1, 1.0 and
1.0E0), then the choice is implementation-dependent.
However, the choice MUST be made so that
given a target RDF datatype and value, the same lexical form
is chosen consistently (e.g., INTEGER 5 and
BIGINT 5 must be mapped to the same lexical form,
as both are mapped to the RDF datatype xsd:integer and
are equal values; mapping one to 5 and the other to
+5 would be in error). The
canonical lexical representation
[XMLSCHEMA2]
MAY be chosen.
(See also: Summary of XSD Lexical Forms)| SQL datatype | RDF datatype | Lexical transformation (informative) |
|---|---|---|
BINARY, BINARY VARYING, BINARY LARGE OBJECT |
xsd:base64Binary |
base64 encoding |
NUMERIC, DECIMAL |
xsd:decimal |
none required |
SMALLINT, INTEGER, BIGINT |
xsd:integer |
none required |
FLOAT, REAL, DOUBLE PRECISION |
xsd:double |
none required |
BOOLEAN |
xsd:boolean |
ensure lowercase (true, false) |
DATE |
xsd:date |
none required |
TIME |
xsd:time |
none required |
TIMESTAMP |
xsd:dateTime |
replace space character with “T” |
INTERVAL |
undefined | undefined |
The translation of INTERVAL is left
undefined due to the complexity of the translation.
[SQL14] describes a translation of
INTERVAL to xdt:yearMonthDuration and
xdt:dayTimeDuration.
In [SQL2],
the precision of many SQL datatypes is not fixed, but left
implementation-defined. Therefore, the mapping to XML Schema datatypes
must rely on arbitrary-precision types such as
xsd:decimal, xsd:integer and
xsd:dateTime. Implementers of the mapping
may wish to set upper limits for the supported precision of these XSD types.
The XML Schema specification allows such partial implementations of
infinite datatypes [XMLSCHEMA2],
and defines specific minimum requirements.
The natural rdf literal is defined for SQL 2008 datatypes. The natural rdf literal should be extended to map vendor-specific datatypes to RDF by behaving as if the table above contained additional rows that associate the SQL datatypes with appropriate RDF-compatible datatypes (e.g., the XML Schema built-in types [XMLSCHEMA2]), and appropriate lexical transformations where required. If there is no appropriate datatype, the value may be cast to string and expressed as an RDF plain literal. Future versions of this specification may define mappings for vendor-specific datatypes or datatypes added to the SQL specification.
Cast to string is an
implementation-dependent function that maps SQL data values
to equivalent Unicode strings. It
is a fallback that can be used to handle vendor-specific and user-defined datatypes
not supported by the R2RML processor. It can be implemented in a number
of ways, including explicit SQL casts (“CAST(value
AS VARCHAR(n))”, where n is an arbitrary
large integer), implicit SQL casts
(concatenation with the empty string), or by employing a
database access API that presents return values as strings. Cast to string is undefined for the following
kinds of SQL datatypes:
collection types, row types, user-defined types without a user-defined
string CAST, reference types whose referenced type does
not have a user-defined string CAST, binary types,
and OPTIONAL further implementation-defined
types.
$Log: sec-10.html,v $ Revision 1.31 2012/01/13 19:34:02 ashok cp §10.html sec-10-AM.html Revision 1.3 2012/01/13 13:35:00 ashok Editorial cleanup Revision 1.2 2012/01/12 19:27:45 eric ~ changes outlined in ericP's proposed changes to §10 Revision 1.1 2012/01/12 19:06:37 eric ~ extracted from R2RML Editor's Draft