Query Languages: Patterns & Structures

Pattern Matching Pattern –a set of syntactic features that must occur in a text segment Types of patterns –Words: string (sequence of characters) in the text –Prefixes: string forming the beginning of a text word (e.g., comput → computer, computation) –Suffixes: string forming the termination of a text word (e.g., ters → computers, painters) –Substrings: string appeared within a text word (allowed word separators) (e.g., tal → talk, metallic & any flow → many flowers)

Pattern Matching (cont.) More Types of Patterns –Ranges: a pair of strings matched any word lying between them in lexicographical order (e.g., held to hold → hoax, hissing) –Allowing errors: retrieving word similar to given word (e.g., flower → flo wer [edit distance = 1]) –Regular expressions: general patterns built up by simple strings and operators (e.g., pro (blem | tein) (s | ε) (0 | 1 | 2)* ) → Problem02, proteins –Extended patterns: classes of characters, conditional expressions, wild characters, combinations

Structural Queries Structural query –mixing contents and structure in queries content constraints (words, phrases, patterns) structural constraints (containment, proximity) and restrictions on structural elements (chapters, sections) Type of structures of text –form-like fixed structure –hypertext structure –hierarchical structure

Structural Queries (cont.) Types of structures formhypertexthierarchical

Fixed Structure Traditional restrictions –documents had a fixed set of fields –each field had some text inside –only rarely the fields appear in any order or repeat –fields were not allowed to nest or overlap –retrieval: specifying a given basic pattern to be found only in a given field Characteristics –reasonable to retrieve text collection having a fixed structure (e.g. mail archive) → inadequate to represent the hierarchical structure such as HTML docs –expansion to relational DB model

Hypertext Hypertext (navigational) –a directed graph where the nodes hold some text and the links represent connections between nodes or between positions inside the nodes Browsing / Searching in hypertext –retrieval from a hypertext: browsing (traversing the hypertext nodes following link → navigational activity) –even in web, one can search by the text contents of the nodes, but not by their structural connectivity –some search engines now allow searching for specific source or destination anchors (but not general structure + content queries)

Hierarchical Structure Hierarchical structure –an intermediate structuring model lying between fixed structure and hypertext –represents a recursive decomposition of the text –a natural model for many text collections, e.g., books, articles, legal documents, structured programs, etc. Hierarchical models –PAT Expressions, Overlapped Lists, List of References, Proximal Nodes, Tree Matching Issues in hierarchical models –static or dynamic structure, restrictions on the structure, integration with text, query language

Query Protocols Query protocols –query language used to query text database –standards intended not for human use but for querying library systems and querying CD-ROMs Some important query protocols –Z39.50 –WAIS –CCL –CD-RDx –SFQL