XDO2: A Deductive Object-Oriented Query Language for XML

1
XDO2 A Deductive Object-Oriented Query Language
for XML

• Wei Zhang1, Tok Wang Ling1, Zhuo Chen1, and
  Gillian Dobbie2
• School of Computing
• National University of Singapore1
•   Department of Computer Science
• University of Auckland
• New Zealand2

2
Outline

• Background and Motivation
• XDO2 Database Example
• XDO2 Language Features
• Simple and compact
• Compact query return format
• Multi-valued variables and aggregate functions
• Separating structure and value
• Not-predicate negation
• Recursion querying
• Comparison with Related Work
• Conclusion and Future Work

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Background - XTree

• XTree 1 is a generalization of and has
  advantages over XPath
• XTree uses to group same level attributes and
  elements together
• One XPath expression defines one variable vs one
  XTree expression defines multiple variables
• XPath cannot define return format. However, XTree
  can define return format
• Multi-valued variables expressed explicitly in
  XTree using vs no syntactic difference in
  XPath
• Element tags (attribute names) separated from
  values in XTree using stru ? value vs use of
  functions to separate structure and value pairs
  in XPath
• In short, XTree is designed to have a tree
  structure while XPath does not

1 Z. Chen, T.W. Ling, M.C. Liu, and G. Dobbie.
XTree for declarative XML querying. In
Proceedings of DASFAA, pages 100-112, Korea, 2004
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Motivation

• Our proposed XDO2 is a novel XML query language
  which is based on XTree and has deductive
  features such as deductive rules and negation,
  and some object-oriented features such as
  inheritance and methods
• Major contributions of XDO2
• Negation supported using not-predicate instead
  of conventional logical negation as used in
  XQuery
• Derived attributes/methods implemented as
  (recursive) deductive rules to deduce new
  properties
• (Multiple) Inheritance enables a subclass object
  to inherit the properties from its superclass
  objects
• Multiple variables in one expression, compact
  return format, explicitly expresses multi-valued
  variables, and separates structure from value

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XDO2 Database Example Company XML data

• ltrootgt
• ltperson pnop1gt
• ltnamegtJohnlt/namegt
• ltaddressgt
• ltstreetgtKing Streetlt/streetgt
• ltcitygtOttawalt/citygt
• lt/addressgt
• ltbirthyeargt1975lt/birthyeargt
• ltsexgtMalelt/sexgt
• lt/persongt
• ltperson pnop2gt
• ltspouse pnop3 /gt
• ltnamegtMikelt/namegt
• ltaddressgt
• ltstreetgtAlbertlt/streetgt
• ltcitygtOttawalt/citygt
• lt/addressgt
• ltbirthyeargt1954lt/birthyeargt
• ltsexgtMalelt/sexgt

• ltnamegtMarylt/namegt
• ltaddressgt
• ltstreetgtAlbertlt/streetgt
• ltcitygtOttawalt/citygt
• lt/addressgt
• ltbirthyeargt1958lt/birthyeargt
• ltsexgtFemalelt/sexgt
• lt/persongt
• ltcompany cnoc1gt
• ltnamegtStarlt/namegt
• ltemployee enoe1 pnop1gt
• ltsalarygt6000lt/salarygt
• lthobbygtTennislt/hobbygt
• lthobbygtSoccerlt/hobbygt
• lt/employeegt
• ltemployee enoe2 pnop2gt
• ltsalarygt4000lt/salarygt
• lthobbygtTennislt/hobbygt
• lt/employeegt

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XDO2 Database Example ORA-SS Schema Diagram
Fig 1 ORA-SS schema diagram
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XDO2 Database Example - Schema Features

• Two derived attributes using rules
• age derived attribute in person class
• bachelor derived attribute in person class
• Inheritance
• Employee class is a subclass of person class and
  inherits all the properties of person class
  (attributes and derived attributes)

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XDO2 Database Example Derived Attribute Age

• Rule R1 defines that the age of a person is 2005
  minus his/her birthyear.
• (R1) p/age a - /root/person p/birthyear
  b, a 2005 - b.
• Notation - is to specify if the right hand
  side (body) is true, then the left hand side
  (head) is true.
• Notation assigns the value of left hand side
  to the right hand side. If the left hand side is
  an object class, then the right hand side binds
  to the object identifier, such as person.
• Single-valued variables are denoted by
  followed by a string literal.

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XDO2 Database Example Derived Attribute Bachelor

• Rule R2 defines that a person is a bachelor if he
  is a male and without spouse.
• (R2) p/bachelor true - /root/person
  p/ sex Male, not(spouse s).
• Notation is to group the same level
  properties (attributes, derived attributes)
  together which are defined under the same parent
  object
• Notation not is to negate the existence of
  enclosed term
• Two Boolean values true and false are reserved

2 T.W. Ling. The prolog not-predicate and
negation as failure rule. New Generation
Computing, 8(1)5-31, 1990
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XDO2 Database Example Inheritance

• The following statement defines that employee
  object class is a subclass of person object
  class.
• employee ISA person by
  employee.pno ISA person.pno
• Notice the first part of the statement defines
  the class inheritance while the second part of
  the statement defines the object inheritance

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XDO2 Database Example Query

• (Eg1) Find the age and salary of all employees
  who are bachelors, with age less than 30, and
  salary larger than 5000.
• /db/youngRichBachelor e/age a, payroll
  s ? /root/company/
• employee e/age a, bachelor true,
  salary s,
• a lt 30, s gt 5000.
• Notation ? is to specify the left hand side is
  used to construct the XML result and the right
  hand side is the query and the conditional parts
• Derived attributes age and bachelor specified in
  rule R1 and rule R2 are used directly in the
  query
• age and bachelor properties are inherited from
  the person class by employee being a subclass of
  person class

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XDO2 Database Example Query Result

• Using the LHS result construction expression of
  example 1, we can generate the following XML data
  result.
• ltdbgt
• ltyoungRichBachelor enoe1gt
• ltagegt29lt/agegt
• ltpayrollgt6000lt/payrollgt
• lt/youngRichBachelorgt
• lt/dbgt
• Note e which binds to the object identifier of
  employee, is used as the value of the result
  element youngRichBachelor

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XDO2 Language Features

• Simple and compact query expression
• (Eg2) Find the year and title of each book, and
  its authors last name and first name.

XDO2 expression /bib/book/_at_year y, title
t, author/last l, first f
XPath expressions book in /bib/book, y in
book/_at_year, t in book/title, author in
book/author, last in author/last, first in
author/first

• One XDO2 expression corresponding to 6 XPath
  expressions
• Using to group the same level attributes,
  elements, and/or methods together to have a tree
  structure.
• book and author are needed in XPath expressions

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XDO2 Language Features

• Compact result format
• (Eg3) List the titles and publishers of books
  which are published after 2000.

XDO2 query expression /result/recentbook/title
t, publisher p ? /bib/book/_at_year y,
title t, publisher p, y gt 2000.
XQuery expression for book in /bib/book, y in
book/_at_year, t in book/title, p in
book/publisher where y gt 2000 return
ltresultgtltrecentbookgtt plt/recentbookgtlt/result
gt

• One XDO2 expression is used for query result
  format while XML element tags is mixed with the
  XPath expressions in XQuery

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XDO2 Language Features

• Multi-valued variables and aggregate functions
• (Eg4) List the titles of the book which has more
  than 1 author.

XDO2 query expression /result/multiAuthorBook/tit
le t ? /bib/book/title t,
author ltagt, ltagt.count() gt 1.
XQuery expression for book in /bib/book, t in
book/title let a in book/author where
count(a) gt1 return ltresultgtltmultiAuthorBoo
kgttlt/multiAuthorBookgtlt/resultgt

• Multi-valued variables are denoted by lt gt
  (list-valued) or (set-valued)
• Object-oriented fashion built-in aggregate
  functions instead of function based fashion

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XDO2 Language Features

• Separating structure from value
• (Eg5) Get sub-element with value John in some
  person element.

XDO2 query expression ele ? /root/person/ele
John.
XQuery expression for b in /root/person/ where
string(b) John return local-name(b)

• Using stru value to separate the structure from
  the value naturally
• In XQuery, built-in functions string() and
  local-name() have to be used to get values and
  structures respectively

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XDO2 Language Features

• Not-predicate negation (nested)
• (Eg6) To retrieve the name of the company where
  every employee has hobby Tennis.

XDO2 query expression /db/allLikeTennisCom n
? /root/company c/name n,
c/not(employee/not(hobby Tennis)).
XQuery expression for c in /root/company where
EVERY e IN c/employee SATISFIES SOME h
IN e/hobby SATISFIES string(h)
Tennis return ltdbgtltallLikeTennisComgtstring(c/n
ame) lt/allLikeTennisComgtlt/dbgt

• XDO2 query expression is much simpler and more
  compact using the not-predicate compared with
  XQuery, which needs keyword EVERY, SOME,
  IN, SATISFIES

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XDO2 Language Features

• Not-predicate negation (sub-tree)
• (Eg7) To retrieve the companies which do not have
  an employee who has sex Male and birthyear 1975.

XDO2 query expression /db/company c ?
/root/company c/not(employee/ sex
Male, birthyear 1975).
XQuery expression for c in /root/company where
NOT (SOME e IN c/employee SATISFIES (e/sex
Male AND e/birthyear 1975)) return
ltdbgtclt/dbgt

• Using the not-predicate on a subtree structure,
  XDO2 expression is more compact while XQuery
  needs NOT, SOME, IN, SATISFIES, AND.

2 T.W. Ling. The prolog not-predicate and
negation as failure rule. New Generation
Computing, 8(1)5-31, 1990
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XDO2 Language Features

• Recursion querying
• (Eg8) Suppose there is a sub-element child
  directly under the person element. The following
  deductive rules define descendants of person.

• p/descendant c - /root/person p/child
  c.
• p/descendant d - /root/person p/child
  c, c/descendant d.
• To retrieve all the descendants of a person with
  pno p1.
• /db/person_p1/descendant d ? /root/person
  p1/descendant d.

• for each descendant of person with pno value
  p1, single-valued variable d will bind to the
  descendants identifier and output the result.
  The recursive rules are recursively used to infer
  all the persons descendants.

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Comparison with Related Work
XDO2 XQuery XTreeQuery F-logic 9 XML_RL 8
Underlying data XML tree XML tree XML tree Object XML tree
Path expression XTree XPath XTree Path expression XTree-like expression
Deductive rule Yes No No Yes Partial
Recursion Recursive rule Recursive function Recursive query Recursive rule Recursive query
Negation Not-predicate Logical negation Logical negation Logical negation Logical negation
Quantification No need Yes Yes Yes Yes
Multi-valued variable Yes No Yes No Yes
Direct structure querying Yes No Yes Yes Yes
Object-oriented features Yes No No Yes No
8 M.C. Liu. A logical foundation for XML. In
CAiSE, pages 568-583, 2002. 9 M. Kifer, G.
Lausen, and J. Wu. Logical foundations of
object-oriented and frame-based languages.
Journal of ACM, 42(4)741-843, 1995.
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Conclusion

• A novel new XML query language XDO2 which is
  based on XTree and has deductive and
  object-oriented features. The relationship
  between XDO2 and XQuery is similar to the
  relationship between Datalog and SQL
• XDO2 queries are simpler and more compact than
  current XML query languages such as XQuery
  because XDO2 is based on XTree, and supports
  (recursive) deductive rules and the
  not-predicate.

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Conclusion Contd

• Important features and contributions of XDO2
• Negation supported using not-predicate instead
  of conventional logical negation as used in
  XQuery
• Derived attributes/methods implemented as
  (recursive) deductive rules to deduce new
  properties
• (Multiple) Inheritance enables a subclass object
  to inherit the properties from its superclass
  objects
• Multiple variables in one expression, compact
  return format, explicit multi-valued variables,
  and separating structure from value are supported
  naturally due to XTree

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Future Work

• How to evaluate the XDO2 queries efficiently?
• Especially those queries involving recursive
  deductive rules and not-predicate.
• Approaches for XDO2 queries evaluation?
• Datalog like bottom-up approach
• Prolog like top-down approach
• Combining both

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References

• 1 Z.Chen, T.W. Ling, M.C. Liu, and G.Dobbie.
  XTree for declarative XML querying. In
  Proceedings of DASFAA, pages 100-112, Korea,
  2004.
• 2 T.W. Ling. The prolog not-predicate and
  negation as failure rule. New Generation
  Computing, 8(1)5-31, 1990.
• 3 T.W. Ling and M.L. Lee and G. Dobbie.
  Semistructured Database Design. Springer, 2005.
• 4 T.W. Ling and W.B.T. Lee. DO2 A deductive
  object-oriented database system. In proceedings
  of the 9th International Conference on Database
  and Expert System Applications, pages 50-59,
  1998.
• 5 M.C. Liu and T.W. Ling. Towards declarative
  XML querying. In Proceedings of WISE, pages
  127-138, Singapore, 2002.
• 6 M.C. Liu, G. Dobbie, and T.W. Ling. A logical
  foundation for deductive object-oriented
  databases. ACM Transactions Database Systems,
  27(1)117-151, 2002.
• 7 M. Kifer, G. Lausen, and J. Wu. Logical
  foundations of object-oriented and frame-based
  languages. Journal of ACM, 42(4)741-843, 1995.
• 8 M.C. Liu. A logical foundation for XML. In
  CAiSE, pages 568-583, 2002.
• 9 M. Kifer, G. Lausen, and J. Wu. Logical
  foundations of object-oriented and frame-based
  languages. Journal of ACM, 42(4)741-843, 1995.
• 10M.C. Liu and T.W. Ling. Towards declarative
  XML quering. In Proceedings of WISE, pages
  127-138, Singapore, 2002.

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The End

• Thank You!
• Any Questions?