Midterm Review: XML Storage and Schema Design, October 31, 2003

1. Lecture 15:Midterm Review Friday, October 31, 2003

2. XML Storage • Most often the XML data is small • E.g. a SOAP message • Parsed directly into the application (DOM API) • Sometimes XML data is large • need to store/process it in a database • The XML storage problem: • How do we choose the schema of the database ?

3. XML Storage Three solutions: • Schema derived from DTD • Storing XML as a graph: “Edge relation” • Store it as a BLOB • Simple, boring, inefficient • Won’t discuss in class

4. Designing a Schema from DTD Design a relational schema for: <!DOCTYPE company [ <!ELEMENT company ((person|product)*)> <!ELEMENT person (ssn, name, office?, phone*)> <!ELEMENT ssn (#PCDATA)> <!ELEMENT name (#PCDATA)> <!ELEMENT office (#PCDATA)> <!ELEMENT phone (#PCDATA)> <!ELEMENT product (pid, name, ((price,availability)|description))> <!ELEMENT pid (#PCDATA)> <!ELEMENT description (#PCDATA)> ]>

5. Designing a Schema from DTD First, constructthe DTD graph: company * We ignorethe order * person product * office phone pid price avail. descr. ssn name

6. Designing a Schema from DTD Next, design therelational schema,using common sense. company * * person product * office phone pid price avail. descr. ssn name Person(ssn, name, office)Phone(ssn, phone)Product(pid, name, price, avail., descr.) Which attributes may be NULL ? (Look at the DTD)

7. Designing a Schema from DTD What happens to queries: FOR$xIN /company/product[description] RETURN <answer> { $x/name, $x/description } </answer> SELECT Product.name, Product.description FROM Product WHERE Product.description IS NOT NULL

8. Storing XML as a Graph Sometimes we don’t have a DTD: • How can we store the XML data ? Every XML instance is a tree • Store the edges in an Edge table • Store the #PCDATA in a Value table

9. 0 1 db 9 2 5 book book publisher 4 8 11 3 6 7 10 title author author title state title author “Complete Guide to DB2” “Morgan Kaufman” “CA” “Chamberlin” “Transaction Processing” “Bernstein” “Newcomer” Storing XML as a Graph Can be ANYXML data(don’t know DTD) Edge Value

10. xdb xbook xauthor xtitle vtitle vauthor Storing XML as a Graph What happens to queries: FOR$xIN /db/book[author/text()=“Chamberlin”] RETURN$x/title db book author title “Chamberlin” Return value

11. Storing XML as a Graph What happens to queries: A 6-way join !!! SELECT vtitle.value FROM Edge xdb, Edge xbook, Edge xauthor, Edge xtitle, Value vauthor, Value vtitle WHERE xdb.source = 0 and xdb.tag = ‘db’ and xdb.dest = xbook.source and xbook.tag = ‘book’ and xbook.dest = xauthor.source and xauthor.tag = ‘author’ and xbook.dest = xtitle.source and xtitle.tag = ‘title’ and xauthor.dest = vauthor.source and vauthor.value = ‘Chamberlin” and xtitle.dest = vtitle.source

12. Storing XML as a Graph Edge relation summary: • Same relational schema for every XML document: Edge(Source, Tag, Dest) Value(Source, Val) • Generic: works for every XML instance • But inefficient: • Repeat tags multiple times • Need many joins to reconstruct data

13. Other XML Topics • Name spaces • XML API: • DOM = “Document Object Model” • XML languages: • XSLT • XML Schema • Xlink, XPointer • SOAP Available from www.w3.org (but don’t spend rest of your life reading those standards !)

14. Research on XML Data Management at UW • Processing: • Query languages (XML-QL, a precursor of XQuery) • Tukwila • XML updates • XML publishing/storage • SilkRoute: silkroute.sourceforge.net • STORED • XML tools • XML Compressor: Xmill • XML Toolkit (xsort, xagg, xgrep, xtransf, etc): xmltk.sourceforge.net • Theory: • Typechecking • Xpath, Xquery containment

15. The Midterm Four questions: • SQL • E/R • Schema design (BCNF/3NF) • XML

16. The Midterm • Open book exam (books,/notebooks,/lectures) • But no computers • No retakes • Your score:Max(midterm-score,100 – 1.2(100 – final-score) • No retakes on final: you *must* take it on Dec. 12

17. 1. SQL • Selection/project/join • Understand well duplicates • Aggregate queries • avoid nested queries when a GROUP BY suffices • Nested queries • More difficult: ANY, ALL, NOT IN • Updates, table creations, views

18. 2. E/R Diagrams • One/many v.s. many/many relationships • Inheritance • Translation to relations • Remember: no table for one/many !

19. 3. Schema Design • What does AB  CD mean ? • Compute {AB}+ • Compute all keys • Decompose in BCNF or 3NF

20. 4. XML • XML: • Basic syntax: elements + attributes • DTDs: elements only • The tree model • Canonical XML view of a relation (<row>...) • XPath • XQuery • Basic principles in publishing/storing data

21. Final Thoughts • Open book • But read the book before the exam • Some question(s) may be hard(er) • Answer first the questions that are easier • The answers should not be very complex

22. Exercises • ABC, CD, DB, DE • Decompose in BCNF General strategy: find X s.t.: X  X+  all attributes

23. ABC, CD, DB, DE BCNF • Try AB+ = ABCDE nope… • Try C+ = CDBE yep…decompose CDBE, CA • Continue in CDBE: try D+ = DBE yep…decompose DBE, DC • Continue in DBE: try D+ = DE yep…decompose DE, DB • Answer: CA, DC, DE, DB Notice: AB is a key, but A and B are separated. Cleary 3NF differs

24. ABC, CD, DB, DE 3NF • Minimal keys: AB, AC, AD • Try C+ = CDBE yes, BUT: D,B part of keysso decompose on C E only:CE, CABD Continue in CABD • Minimal keys: AB, AC, AD • Try A+ = A nope… Try B+ = B nope…Try AB+ = ABCD nope…Try C+ = CDB yes, but B, D part of keysTry D+ = DB again B part of a key Try others… nope. • Answer: CE, CABD. Notice: DE, DABC also OK