Re: RIF in terms of SQL views and Datalog:

:)

Juan Sequeda
+1-575-SEQ-UEDA
www.juansequeda.com


On Tue, Jul 27, 2010 at 8:26 AM, Eric Prud'hommeaux <eric@w3.org> wrote:

> Just so we can all speak each others' languages, here's a definition
> of a view in SQL, Datalog (Horn Rules without functions) and RIF¹.
> Imagine two tables, Employees and Departments:
>
>  Employees(emp_id, fname, lname, dept)
>  Departments(dpt_id, deptname, manager)
>
>  CREATE TABLE Employees(emp_id INT PRIMARY KEY, fname STRING, lname STRING,
> dept INT)
>  CREATE TABLE Departments(dpt_id INT PRIMARY KEY, deptname STRING, manager
> INT)
>
> In SQL, we can create a view of employees and their managers,
> EmpManager:
>
>  CREATE VIEW EmpManager AS
>    SELECT (e.emp_id, e.fname, e.lname, m.fname, m.lname)
>      FROM Employees AS e
>      JOIN Departments AS d ON d.dpt_id=e.dept
>      JOIN Employees AS m ON m.emp_id=d.manager
>
> We can write this in Datalog:
>
>  EmpManager(EMP_ID, E_FNAME, E_LNAME, M_FNAME, M_LNAME) :-
>    Employees(EMP_ID, E_FNAME, E_LNAME, DEPT),
>    Departments(DEPT, _, MANAGER),
>    Employees(MANAGER, M_FNAME, M_LNAME, _)
>
> or in RIF's n-ary predicate syntax:
>
>  Document(
>    Base(<http://mydb.example/#>)
>    Group (
>      Forall ?emp_id ?e_fname ?e_lname ?e_dept
>             ?e_dept, ?deptName, ?manager
>             ?manager ?d_fname ?d_lname ?d_dept (
>        EmpManager(?emp_id, ?e_fname, ?e_lname, ?m_fname, ?m_lname) :-
>          Employees(?emp_id, ?e_fname, ?e_lname, ?dept),
>          Departments?(dept, ?deptName, ?manager),
>          Employees(?manager, ?m_fname, ?m_lname, m_dept)
>      )
>    )
>  )
>
> ¹ If Employees or Departments did not have unique keys (labeled
>  "PRIMARY KEY" in the SQL definitions), these wouldn't be strictly
>  identical views as the SQL operational semantics preserves
>  cardinality over multisets, while the others use set semantics for
>  their predicates and specifically avoid an operational semantics.
>
> For interested parties, the RIF semantics are defined at
>  http://www.w3.org/TR/rif-bld/#Direct_Specification_of_RIF-BLD_Semantics
>
> --
> -ericP
>
>