Chapter 9 - SQL/XML

Question 1

What is the motivation and the different scenarios of SQL/XML integration?

Answer 1

• goals: flexible exchange between relational data and XML, reliable XML management
  
  • > native XML DBMS not mature enough
• simple reuse of OR capabilities is not sufficient
  
  • > LOBs for XML storate: coarse granularity
  • > decompose into tables: complex and inefficient
• hybrid relational /XML data management: storage, acces, query, APIs
  
  • > view/ access relational as XML/ XML as relational

Question 2

Overview: XML Data Type

Answer 2

• built-in SQL type for XML -> instances of XQuery Data Model
  
  • > optimized internal representation
  • > used in SQL/XML functions / store XML fragments in columns
• allows restrictions of XML value: XML(SEQUENCE), XML(DOCUMENT), XML(CONTENT)
• conversion from/to character string/BLOB: XMLPARSE( ), XMLSERIALIZE( )

Question 3

Overview: XMLCAST to convert between XML and SQL

Answer 3

• SQL -> XML : SQL predefined type value to XQuery atomic
• XML -> SQL: atomic or nodes (fn:data) to SQL types
• parse and serialize convert between node and sql varchar

Question 4

Overview: Schema and Validation

Answer 4

• Register schema in the DBMS for further referencing
• fnction XMLVALIDATE ( ACCORDING TO XMLSCHEMA ID )
  
  • > annotates xml values and validate them

Question 5

Overview: Constructor functions

Answer 5

• Generating values of XML data type within SQL
  
  • > one function for each node kind: XMLELEMENT, XMLATTRIBUTE, XMLTEXT…
  • > sequence of values: XMLFOREST, XMLCONCAT
  • > aggregation over sets of tuples: XMLAGG
• functions take content as arguments, alloing arbitrary nesting
• XMLAGG: used in combination with GROUP BY, evaluate content exprssion
  
  • > produces a sequence of XML values, one for each row in the group
  • > optional ORDER BY
• Dynamic retrieval of SQL data into XML -> publishing relational data as XML

Question 6

How can XML data be manipulated in SQL?

Answer 6

XQUERY support:

• Inverse of constructors: publish XML as relational
• extract portions of XML data stored in DBMS

XML Query function: evaluates XQuery expression

• > argument passing: SQL values inside the query (also XML data type)
  • -> Use XQuery vars (e.g. $dept) with PASSING BY REF as dept

-> returns sequence

XMLTABLE function: transofrms XML data into table format

• > ROW pattern: XQuery expression, path item in result sequence will be a row
• > Column pattern: extract values from row item using path expressions
• > XMLQUERY syntax, with additional COLUMNS ( PATH )

Predicates on XML type: IS DOCUMENT, IS CONTENT, IS VALID, XMLEXISTS

Question 7

How does the mapping of an SQL schema to XML work?

Answer 7

TABLES: approach are single element and sequence of elements

• > to map schema or catalog, use its name as root node, with tables inside
• > produces XML document with table contents + XML schema