XML to Relational Mapping

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XML to Relational Mapping
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XML-to-Relational Mapping
XML Translation Layer
Tuples
Relational Database System
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Where/How to Store XML Data ?

• File system
• OODBMS
• Semistructured DBMS Lore, etc.
• XML DBMS eXcelon, Tamino, etc.
• RDBMS/ORDBMS
• CLOB(Character Large Object) column
• Decomposing (Shredding) into tuples
• XML Type

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Approaches to XML Data Storage Using an RDBMS

• Predefined set of table schema
• Automatic generation of table schema for given
  DTD
• Data mining approach

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Predefined Schema

• simple ad-hoc schemes
• requires no input by the user or by the system
  administrator
• works in the absence of type (e.g., DTD)
• does not involve any analysis of the XML data
• many variations possible
• Edge-inlining
• XRel
• XParent

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Predefined SchemaFlor99
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XML Data Model

• Ordered labeled graph
• Each XML element is represented by a node in the
  graph the node is labeled with the oid of the
  XML object.
• Element-subelement relationships are represented
  by edges in the graph and labeled by the name of
  the subelement.
• In order to represent the order of subelements of
  an XML object, the outgoing edges of a node in
  the graph are also ordered.
• Values (e.g., strings) of an XML document are
  represented as leaves in the graph.

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Example XML Data
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XML Data Model
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Mapping XML Data into Relational Table

• Mapping Edges
• Edge Approach
• Binary Approach
• Universal Table Approach
• Mapping Values
• Separate Value Tables
• Inlining
• 3 x 2 6 possible mapping schemes

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Edge Approach

• Store all edges of the XML document in a single
  table Edge Table

Edge (source, ordinal, name, flag, target)
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Edge Approach (Contd)

• The index on the source column is useful for
  forward traversals needed to reconstruct a
  specific object given its oid.
• The index on name, target is useful for
  backward traversals
• e.g.) find all objects that have a child
  named John.

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Binary Approach

• Group all edges with the same label into one
  table
• This approach corresponds to a horizontal
  partitioning of the Edge Table, using name as the
  partitioning attribute.

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Universal Table Approach

• Generate a single Universal Table to store all
  the edges.
• The Universal Table corresponds to the result of
  a full outer join of all Binary Tables.
• The Universal Table has many fields which are set
  to NULL.
• This Universal Table is denormalized.

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Separate Value Tables

• Store values in the separate Value Tables.
• A separate Value Table for each conceivable data
  type
• e.g., integer, date, ref, string, etc.

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Inlining

• Store values and attributes in the same table
• e.g., binary inlining approach

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Modified Edge-Inlining Approach

• XML Data Model ordered, labelled tree
• Node id (i.e., source) assigned in preorder
  traversal of XML tree
• 1 Table
• Node id (source)
• Tag name (name)
• Text (PCDATA)
• Parent Node id (tagging)
• Path (optional)
• Type (element or attribute)
• Level
• Doc id

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Automatic Schema Generation out of DTDShan 99

• Assumption
• XML document conforms to a scheme (DTD)
• Transformation Approach
• Three Techniques
• Basic, Shared, Hybrid Inlining
• DTD ? DTD Graph ? Element Graph ? Table Schema

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DTD to Relational Schema

• Naïve Approach
• Each Element gt Relation
• Each Attribute of Element gt Column of Relation
• Connect elements using foreign keys
• Fragmentation problem
• Too many relations
• Requires many joins in query processing

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Naïve Approach Example
lt!ELEMENT author (name, address)gt lt!ATTLIST
author id ID REQUIREDgt lt!ELEMENT name
(firstname?, lastname)gt lt!ELEMENT firstname
(PCDATA)gt lt!ELEMENT lastname (PCDATA)gt lt!ELEME
NT address ANYgt
author (authorID integer, id string) name
(nameID integer, authorID integer) firstname
(firstnameID integer, nameID integer, value
string) lastname (lastnameID integer, nameID
integer, value string) address (addressID
integer, authorID integer, value string)
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Inlining

• Inlining
• solves fragmentation problem
• Basic inlining as many descendents of an element
  as possible into a single relation
• Shared inlining the nodes with an in-degree of 1
  (DTD graph)
• Hybrid
• Basic Shared
• inlining the nodes with an in-degree gt 1 (DTD
  graph)
• Issues
• set-valued attribute
• recursion

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DTD
?
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DTD Graph
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Element Graph for editor
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Basic Inlining Technique
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Shared Inlining Technique
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Hybrid Inlining Technique

• Same as Shared Inlining except
• inlines some elements that are not inlined in
  Shared
• inlines elements with in-degree greater than one
• that are not recursive or
• reach through a node

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Data Mining Approach

• Data Pattern Analysis
• Wang Lis Semistructured Data Mining Algorithm
• STORED(Semistructured To Relational Data)
• a declarative language for description of
  XML-to-Relational Mapping
• Overflow Mapping
• definition for the objects not mapped to
  relational data
• overflow graph
• stored in the object repository of semistructured
  data

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Data Pattern Analysis
STORED Mapping
XML Data
Algo
Overflow Mapping