Query Languages

Aswin Yedlapalli

XML Query data model

• Document is viewed as a labeled tree with nodes

• Successors of node may be : - an ordered sequence of nodes (eg.

for sub elements).- an unordered set of nodes (eg. For attributes).

• Compatible with XML schemas

Comparison of XML and semi structured data

• Similarities:- both are best described by a labeled

graph.- both are schema-less self describing.

• Differences:- XML is ordered; semi structured

data is unordered.- XML can mix text and elements

Required features for a Query Language

• Expressive power- The Query language must be at least as

expressive as SQL on relational data.- The Query language should have the

ability to restructure data.- The Query language should be able to

navigate data with arbitrary nesting.• Semantics

- It is very important in a query language for query transformation and optimization.

• Compositionality

- Our queries must remain in the same data model. They cannot take data in one model and produce output in another model.

• Schema

- when structure is defined, a query language should be exploited for optimization, type checking etc.,

Query languages

• For semi structured data- Lorel (Lightweight Object REpository Language)- UnQL (Unstructured Query Language)-StruQL, MSL, W3QL, WebSQL, Weblog, etc.,

• For XML- XML-QL (XML Query Language)- XSLT & structural recursion.- XML Query Algebra.

Formal Semantics

• Given query Q = SELECT E[X1,……. Xn] FROM F WHERE Cand database DB

Answer: (Q,DB) is defined in two steps:

–Step 1: compute all bindings:

•Cij are node oids or atomic values

•Must satisfy paths in F

•Must satisfy conditions in C

–Step 2: answer is E[C11, …, C1n] …

E[Cm1, …, Cmn]

• When E has nested sub queries, apply semantics recursively

• Note: so far we have dealt with an unordered model

• –What do we need to do for order ?

• •Complexity: PTIME in |DB| (not in |Q|).

LOREL

• Minor syntactic differences in regular pathexpressions (% instead of _, # instead of _*)

• Common path convention SELECT biblio.book.author

FROM biblio.bookWHERE biblio.book.year = 1999

Becomes

SELECT X.author

FROM biblio.book X

WHERE X.year = 1999

Lorel

• Query language of LORE system adapts OQL to semi structured data.

Select X.title

from bib.article X

where “tova milo” in X.author

returns {title: “type inf…”}

Features of Lorel

• Differences with typed query languages- performs implicit coercions.- deals with missing attributes.- deals with set valued attributes.

eg., x.year > 1998 may have several years.

• Select clause creates new nodes.• Allows for nested queries.• Allows for regular path expressions.

UnQL (Unstructured Query language)

• UnQL is an extension of basic LOREL.• UnQL does not make use of coercion unlike

LOREL.• “Where” clause contains 2 kinds of constructs.

- generators; variables are bound via patterns.

- conditions; as in LOREL• “from” clause is not needed as variables are bound

in patterns.

UnQL Queries

• Eg., Select title:Twhere {bib:article:{title:T, year:Y}}in db,

y>1998.• Root of the database is explicitly represented: db• UnQL queries can be rewritten in LOREL.

The equivalent LOREL for the above query is: select title:Tfrom bib.article A, A.title T, A.year Y

where Y>1998.

Additional features of LOREL

• Label variables

- can combine “schema” and “data” information.

- can turn tables to data and vice-versa.

- perform group-by operations.

• Can match variables with regular expressions.

References

• Managing XML and semi structured data – Lecture series by Prof. Dan Suciu.

• website:www.cs.washington.edu/homes/suciu/COURSES/590DS/