OpenLink Virtuoso - RDF Views

Exposing SQL Data as RDF

OpenLink Virtuoso Universal Server

Cross-platform server for SQL, XML and RDF data management
Includes
- Virtual database engine
- Web Services deployment platform
- Web application server
- SPARQL support and an RDF data store tightly integrated with its relational storage engine

Virtuoso RDF Views

Expose pre-existing relational data as virtual RDF graphs
Available for querying through SPARQL or SPASQL (SPARQL embedded in SQL)
No physical regeneration of relational data

Northwind Demo Database

Tables include:
Demo.demo.Customers

Demo.demo.Customers

Demo.demo.Orders:

Demo.demo.Orders

Northwind Demo Database: RDF View Definition Extract

Demo.demo.Customers:

Northwind RDF View Definition

prefix northwind: <http://www.openlinksw.com/schemas/northwind#>
...
create iri class northwind:Customer
<http://^{URIQADefaultHost}^/Northwind/Customer/%U#this>
(in customer_id varchar not null)
...
alter quad storage virtrdf:DefaultQuadStorage
...
from Demo.demo.Customers as customers
from Demo.demo.Orders as orders ...{

create virtrdf:NorthwindDemo
as graph iri ("http://^{URIQADefaultHost}^/Northwind") {
...
northwind:Customer(customers.CustomerID) a foaf:Organization
as virtrdf:Customer-CustomerID ;
northwind:companyName customers.CompanyName as ... ;
...
northwind:fax customers.Fax as virtrdf:Customer-fax .

northwind:Customer(orders.CustomerID)
northwind:has_order northwind:Order(orders.OrderID)
as virtrdf:Order-has_order .
... }}

Northwind Demo Database: RDF View Definition Extract

Demo.demo.Orders:

Northwind RDF View Definition

prefix northwind: <http://www.openlinksw.com/schemas/northwind#>
...
create iri class northwind:Order
<http://^{URIQADefaultHost}^/Northwind/Order/%d#this>
(in order_id integer not null) .
...
alter quad storage virtrdf:DefaultQuadStorage
...
from Demo.demo.Customers as customers
from Demo.demo.Orders as orders ... {

create virtrdf:NorthwindDemo
as graph iri ("http://^{URIQADefaultHost}^/Northwind") {
...
northwind:Order (orders.OrderID) a northwind:Order
as virtrdf:Order-Order ;
northwind:orderDate orders.OrderDate
as virtrdf:Order-order_date ;
northwind:requiredDate orders.RequiredDate
as virtrdf:Order-required_date ;
...

northwind:has_customer northwind:Customer (orders.CustomerID)
as virtrdf:Order-order_has_customer
northwind:has_employee northwind:Employee (orders.EmployeeID)
as virtrdf:Order-order_has_employee ;
... } }

Northwind Demo Database: Customer Table to RDF Entity Mapping

Customer Table to RDF Entity Mapping

Customer Table to RDF Entity Mapping Diagram

Northwind Demo Database: RDF Browser View - Customer

RDF Browser View: Customer

Northwind Demo Database: RDF Browser View - Order

RDF Browser View: Order

Example: Product Portfolio - Table View

Product Portfolio: Table View

Example: Product Portfolio - RDF View

Product Portfolio: RDF View

Meta-Schema Language

RDF Views =
- Virtuoso RDF Meta-Schema +
- Meta-Schema Language
MSL =
- A domain specific, declarative language for mapping a logical SQL data model to a conceptual RDF data model

MSL Building Blocks

Main building blocks:
- Quad map patterns
- IRI classes
- Literal classes
Organizational enhancements
- Group map patterns
- Quad storage
- Naming

Organizational Enhancements

Make it easier to administer large sets of quad map patterns
Group map patterns - group together map patterns sharing a common graph
Quad storage - groups together group map patterns as a named set
Both allow map patterns to be altered/deleted individually or as a group

Quad Map Pattern

Basic unit of meta schema
Defines transformation from one set of relational columns into triples that match one SPARQL graph pattern
Comprises 4 declarations of quad map values - each calculates a triple field value from the SQL data

Quad Map Pattern - Example

graph <http://www.openlinksw.com/oplweb>
subject p:product_iri (oplweb2.oplweb.product.product_id)
predicate p:description
object oplweb2.oplweb.product.product_description

Alternative SPARQL-Style Notation

graph <http://www.openlinksw.com/oplweb>
{
p:product_iri (oplweb2.oplweb.product.product_id) p:description
oplweb2.oplweb.product.product_description .
}

Named Quad Map Patterns

Group Map Patterns

Quad map patterns for the same graph can be grouped together into a group map pattern

Group Map Patterns

Quad Storage

Quad storage = a named set of quad patterns
Compartmentalizes the RDF to SQL mapping
Contained quad patterns can be manipulated en-bloc
- drop quad storage storage-name
- create quad storage storage-name { quad-map declarations }
- alter quad storage storage-name { quad-map declarations or drop commands }
Map patterns can only be created inside a quad storage definition

IRI Classes - Creating IRIs from Keys

IRI class - constructs a subject IRI for each primary key column value
Defines how key values (for atomic or compound key) are combined into / decomposed from an IRI string

sparql
prefix prd: <http://www.openlinksw.com/schemas/oplweb/product#>
create iri class prd:product_iri
  "http://www.openlinksw.com/oplweb/product#%s"
  (
  in product_id varchar not null
  ) .

IRI Classes - Format Strings

Sprintf-style format string for performing conversion
Other format specifiers supported. E.g. %d
The reverse conversion is inferred automatically

IRI Classes - Complex Conversions

For more complex conversions, functions can be specified that assemble and disassemble an IRI from/into its constituent parts
create iri class prd:product_iri using
function oplweb2.oplweb.product_uri (in id varchar)
returns varchar,
function oplweb2.oplweb.product_uri_inverse (in id_iri varchar)
returns varchar .
Functions product_uri and product_uri inverse would be defined elsewhere in Virtuoso/PL

Literal classes / Identity classes

Non-key columns normally form the object of a triple
Object component of an RDF triple can be an IRI or an RDF literal
Literal classes create RDF literals from non-key column values
Identity class = special form of literal class
- Converts a SQL VARCHAR value into an untyped literal
- Converts values of other SQL data types to an appropriate XML Schema typed literal (e.g. xsd:integer or xsd:date)
- Need not be defined explicitly
- Invoked implicitly when a column name is used directly as the object in a quad map pattern

Quad Pattern Construction Rules

An RDF View is defined by combining the building blocks described so far into a collection of quad patterns
Each quad pattern can consist of:
- Subject: an IRI class or constant IRI
- Predicate: a constant IRI or an IRI class
- Object: an IRI class, literal class, literal IRI or a scalar
- Graph: a constant IRI or an IRI class
- Filter: an optional SQL search condition used to filter rows

A Simple RDF View Definition

prefix p: http://www.openlinksw.com/schemas/oplweb/product#
prefix pc: <http://www.openlinksw.com/schemas/oplweb/product_category#>

alter quad storage virtrdf:DefaultQuadStorage
from oplweb2.oplweb.product as product_tbl
from oplweb2.oplweb.product_category as product_category_tbl
{
create virtrdf:product_portfolio as graph <http://www.openlinksw.com/oplweb/>
  {
  p:product_iri(product_tbl.product_id) a p:Product
    as virtrdf:product_product_id ;
  p:details product_tbl.long_description
    as virtrdf:product_long_description ;
  p:product_category pc:product_category_iri(product_tbl.product_cat_id)
    as virtrdf:product_product_cat_id .

  pc:product_category_iri(product_category_tbl.product_cat_id)
    a pc:ProductCategory
    as virtrdf:product_category_product_cat_id ;
  ...
  } .
} .

Additional Meta Schema Language Features

Table aliases
SQL selection and SQL joins through filters

Table Aliases

Provide a concise alternative to fully qualified table names in map patterns
Declared with a statement such as:
from oplweb2.oplweb.product as product_tbl
Declared once at the start of the storage declaration
- Because quad map patterns usually share a common set of source tables
- Shared between all the quad maps defined in the storage declaration

SQL Selection & Joins Through Filters

SQL Projection
- Not always desirable to expose all the columns in a table or view
  - Security reasons etc.
  - Key column value may have no meaning beyond being a unique identifier which is transformed into an IRI
- Obvious - simply :
  - Omit defining quad map patterns for the relevant columns
  - Omit property definitions for these columns in the RDFS class for the table

SQL Selection

A quad map pattern can include a condition
- i.e. a boolean SQL expression
- Filters out unwanted rows of source data
- SQL expression includes placeholders where a table reference should be printed

SQL Selection

Associating Filters with Table Aliases

An alternative to specifying filter conditions directly in quad patterns is to specify them alongside a table alias
from DB.DBA.SYS_USERS as user where (^{user.}^.U_IS_ROLE = 0)
from DB.DBA.SYS_USERS as group where (^{group.}^.U_IS_ROLE = 1)
All map patterns using the alias are subject to the same filter condition
If several aliases have associated conditions, the WHERE clause of the generated SQL contains a combination of all relevant conditions

SQL Joins

As well as filtering, a quad map pattern condition can also specify join criteria
Multiple conditions can be associated with a table alias
For complex joins a quad map pattern can reference a SQL view joining multiple tables
from DB.DBA.SYS_USERS as user where (^{user.}^.U_IS_ROLE = 0)
from DB.DBA.SYS_USERS as group where (^{group.}^.U_IS_ROLE = 1)
from DB.DBA.SYS_USERS as account
from user as active_user where (^{active_user.}^.U_ACCOUNT_DISABLED = 0)
from DB.DBA.SYS_ROLE_GRANTS as grant
  where (^{grant.}^.GI_SUPER = ^{account.}^.U_ID)
  where (^{grant.}^.GI_SUB = ^{group.}^.U_ID)
  where (^{grant.}^.GI_SUPER = ^{user.}^.U_ID)