In the simplest sense, any relational schema can be rendered into RDF by converting all primary keys and foreign keys into IRI's, assigning a predicate IRI to each column, and an rdf:type predicate for each row linking it to a RDF class IRI corresponding to the table. Then a triple with the primary key IRI as subject, the column IRI as predicate and the column's value as object is considered to exist for each column that is neither part of a primary or foreign key.

Strictly equating a subject value to a row and each column to a predicate is often good but is too restrictive for the general case.

  • Multiple triples with the same subject and predicate can exist.

  • A single subject can get single-valued properties from multiple tables or in some cases stored procedures.

  • An IRI value of a subject or other field of a triple can be composed from more than one SQL value, these values may reside in different columns, maybe in different joined tables.

  • Some table rows should be excluded from mapping.

Thus in the most common case the RDF meta schema should consist of independent transformations; the domain of each transformation is a result-set of some SQL SELECT statement and range is a set of triples. The SELECT that produce the domain is quite simple: it does not use aggregate functions, joins and sorting, only inner joins and WHERE conditions. There is no need to support outer joins in the RDF meta schema because NULLs are usually bad inputs for functions that produce IRIs. In the rare cases when NULLs are OK for functions, outer joins can be encapsulated in SQL views. The range of mapping can be described by a SPARQL triple pattern: a pattern field is a variable if it depends on table columns, otherwise it is a constant. Values of variables in the pattern may have additional restrictions on datatypes, when datatypes of columns are known.

This common case of an RDF meta schema is implemented in Virtuoso, with one adjustment. Virtuoso stores quads, not triples, using the graph field (G) to indicate that a triple belongs to some particular application or resource. A SPARQL query may use quads from different graphs without large difference between G and the other three fields of a quad. E.g., variable ?g in expression GRAPH ?g {...} can be unbound. SPARQL has special syntax for "graph group patterns" that is convenient for sets of triple patterns with a common graph, but it also has shorthands for common subject and predicate, so the difference is no more than in syntax. There is only one feature that is specific for graphs but not for other fields: the SPARQL compiler can create restrictions on graphs according to FROM and FROM NAMED clauses.

Virtuoso Linked Data Views should offer the same flexibility with the graphs as SPARQL addressing physical triples. A transformation cannot always be identified by the graph used for ranges because graph may be composed from SQL data. The key element of the meta schema is a "quad map pattern ". A simple quad map pattern fully defines one particular transformation from one set of relational columns into triples that match one SPARQL graph pattern. The main part of quad map pattern is four declarations of "quad map values ", each declaration specifies how to calculate the value of the corresponding triple field from the SQL data. The pattern also lists boolean SQL expressions that should be used to filter out unwanted rows of source data (and to join multiple tables if source columns belong to different tables). There are also quad map patterns that group together similar quad patterns but do not specify any real transformation or even prevent unwanted transformations from being used, they are described in "Grouping Map Patterns" below.

Quad map values refer to schema elements of two further types: "IRI classes" and "literal classes".

[Note] Note

In SQL, adding a new view can not break anything. This is because SQL lacks the ability of querying "everything" so data sources are always specified. This is not true for SPARQL, so please treat any metadata manipulation as potentially destructive operation. If an RDF storage is supposed to be used by more than one application then these applications should be tested together, not one after other, and they should be installed/upgraded on live database in the very same order as they were installed/upgraded on instrumental machine during testing. Always remember that these applications share RDF tables so they may interfere.

An IRI class declares that a column or set of columns gets converted into a IRI in a certain way. The conversion of this sort can be declared revertible (bijection) so an IRI can be parsed into original SQL values; this is useful when some equality of an IRI constant and a calculated IRI can be replaced with an equality of a parse result of a constant and an SQL column that is index criteria or simply faster. In addition, the SPARQL optimizer will eliminate redundant conversions if one IRI class is explicitly declared as a subclass of another. The most flexible declaration for conversion consists of specifying functions that assemble and disassemble from IRI into its constituent parts. This is overkill for typical conversions so it is possible to specify only one sprintf-style format string such that sprintf() SQL function will print an IRI using this format and sprintf_inverse() will be able to parse it back.

The use of sprintf_inverse() assumes that the format does not contain fragments like '%s%s' that make it impossible to separate parts of IRI from each other.

In the following, we shall map the Virtuoso users and user roles system tables into the SIOC ontology.

create iri class oplsioc:user_iri "http://myhost/sys/user?id=%d"
  (in uid integer not null) .
create iri class oplsioc:group_iri "http://myhost/sys/group?id=%d"
  (in gid integer not null) .
create iri class oplsioc:membership_iri
  (in super integer not null, in sub integer not null) .
create iri class oplsioc:dav_iri "http://myhost%s"
  (in path varchar) .

These IRI classes are used for mapping data from the DB.DBA.SYS_USERS and DB.DBA.SYS_ROLE_GRANTS system tables that are defined in Virtuoso as follows:

create table DB.DBA.SYS_USERS (
  U_ID                integer not null unique,
  U_NAME              char (128) not null primary key,
  U_IS_ROLE           integer default 0,
  U_FULL_NAME         char (128),
  U_E_MAIL            char (128) default ",
  U_ACCOUNT_DISABLED  integer default 1,
  U_DAV_ENABLE        integer default 0,
  U_SQL_ENABLE        integer default 1,
  U_HOME              varchar (128),
. . .

Single record in DB.DBA.SYS_USERS corresponds to a plain user or a group (role). Users and roles are collectively named "grantees". Thus a role may be granted to another role or to a user account. A role grant may be direct (explicit) or assigned by recursion.

create table SYS_ROLE_GRANTS (
  GI_SUPER   integer,
  GI_SUB     integer,
  GI_DIRECT  integer default 1,
. . .
  primary key (GI_SUPER, GI_SUB, GI_DIRECT));

One IRI class usually corresponds to one ontology class, because similar things are usually called similarly. One may wish to use identifiers of ontology classes as identifiers of related IRI classes, to not remember double number of names, e.g. create IRI class mybank:XpressXfer for subjects that will have rdf:type property mybank:XpressXfer made by mapping. That is technically possible but proven to become inconvenient and misleading as application evolves. While RDF types tend to persist, IRI classes may change over time or same subject may get more than one name via more than one IRI class, say, for exports to different systems. It is found to be more convenient to compose names of IRI classes by adding some common prefixes or suffixes to RDF classes (or to table names), say, write create IRI class mybank:XpressXfer_iri .

A "literal class" declares that a column or set of columns gets converted into a literal instead of an IRI. More precisely, the result of conversion can be IRI_ID so it represents an IRI, but in current version of Virtuoso this is supported only for some internal built-in literal classes, not for classes declared by the user. So for user-defined literal class the result of the conversion is an RDF literal even if it is a string representation of a valid IRI.

In any case, a literal class can be used only in quad map values of O fields, because Virtuoso does not support literal values as subjects.

A special case of literal class is the identity class that converts a value from varchar column into an untyped literal and value from column of any other SQL datatype into a typed literal with type from XMLSchema set, i.e. xsd:integer , xsd:dateTime and so on. Columns of types ANY and IRI_ID are not supported.

The SPARQL optimizer knows that RDF literal types are pairwise disjoint so literal classes that produce literals of different types are known to be pairwise disjoint. The optimizer will replace a join on two disjoint literal classes with an empty statement, to simplify the resulting query.

The following declaration of quad map pattern is self-explanatory. The line for object uses identity literal class so there's no need to specify its name.

graph      <http://myhost/sys>
subject    oplsioc:user_iri (DB.DBA.SYS_USERS.U_ID)
predicate  foaf:email

The description language also supports SPARQL-style notation that contains less keywords and eliminates duplicate graphs, subjects and predicates. The following add two patterns with constant graph IRI <http://myhost/sys> and subjects are made from column DB.DBA.SYS_USERS.U_ID by oplsioc:user_iri .

graph <http://myhost/sys>
    oplsioc:user_iri (DB.DBA.SYS_USERS.U_ID)
      a sioc:user ;
      oplsioc:name DB.DBA.SYS_USERS.U_FULL_NAME .

In real applications, quad map patterns should be named, for schema manipulation and keeping debug info readable. Thus it is much better to rewrite the previous example as

create virtrdf:SysUsers as graph <http://myhost/sys>
    oplsioc:user_iri (DB.DBA.SYS_USERS.U_ID)
      a sioc:user
          as virtrdf:SysUserType-User;
      oplsioc:name DB.DBA.SYS_USERS.U_FULL_NAME
          as virtrdf:SysUsersFullName .

Using these names, one may later write, say, drop quad map virtrdf:SysUserType-User .

One name, virtrdf:DefaultQuadMap is reserved. It is an internal quad map pattern used to access "native-form" quads from DB.DBA.RDF_QUAD :

create virtrdf:DefaultQuadMap as
graph rdfdf:default-iid-nonblank (DB.DBA.RDF_QUAD.G)
subject rdfdf:default-iid (DB.DBA.RDF_QUAD.S)
predicate rdfdf:default-iid-nonblank (DB.DBA.RDF_QUAD.P)
object rdfdf:default (DB.DBA.RDF_QUAD.O)

IRI classes from rdfdf:... namespace are also reserved.

The previous example actually contains three map patterns, not two. The name virtrdf:SysUsers refers to a "group map pattern " that does not define any real transformation of relational data into RDF but helps organize quad map patterns into a tree. Group may contain both quad map patterns and other groups. A group can be manipulated as a whole, e.g. drop quad map virtrdf:SysUsers will remove all three map patterns.