CQL – a Common Query LanguageMike Taylor CQL – a Common Query Language 1. What CQL is 2. Motivation 3. Examples and explanation 4. Applications 5. Implementation.

CQL – a Common Query Language Mike Taylor <mike@indexdata.com>

CQL – a Common Query Language

1.What CQL is

2.Motivation

3.Examples and explanation

4.Applications

5.Implementation

Chapter 1: What CQL is

● CQL is a query language:– For humans to type– For query forms to generate– For translating other languages into

● The only query language of SRW/SRU

● Also applicable in other contexts:– Z39.50 (instead of the Type-1 Query)– Query boxes for web searches

Chapter 2: Motivation

Most query languages fall into one of two camps:

● Complex and powerful, but cryptic and hard to learn– SQL, Prefix Query Format (PQF), XML Query

● Easy to learn and use, but lacking in power– Google, AltaVista, CCL

CQL aims to “make simple queries easy, and complexqueries possible” (to paraphrase Larry Wall, of Perl)

Learning curves for query languages

Power of query that can be expressed

in learn

Google

in learn

Google

Chapter 3: Examples and explanation

Important concepts

● Simple terms● Quoting● Booleans● Parentheses● Pattern matching● Word anchoring● Indexes● Prefixes● Context sets● Relations

Esoteric concepts

● Proximity● Relation modifiers● Boolean modifiers● Prefix mapping

CQL features: simple terms

Here are some perfectly good CQL queries:

● fish

● Churchill

● dinosaur

● comp.sources.misc

CQL features: quoting

Double-quote marks remove the special meanings ofspecial characters like space (which otherwise separatestokens) and of keywords such as “and” and “or”.

● "dinosaur"● "the complete dinosaur"● "ext–>u.generic"● "and"● "the \"nuxi\" problem"

(Backslash removes the special meaning of followingdouble-quote characters.)

CQL features: booleans

The keywords “and” and “or” are boolean operators.The keyword “not” is an and-not binary operator.There is no unary negation operator. Case is notsignificant, so “AND” and “aNd” also work.

● dinosaur or bird● dinosaur not reptile● dinosaur and bird and reptile● dinosaur and bird or dinobird● dinosaur not theropod not ornithischian

CQL features: boolean precedence

The “and”, “or” and “not” booleans all have equalprecedence and are evaluated left-to-right.

● dinosaur and bird or dinobirdMEANS

(dinosaur and bird) or dinobird

● dinosaur or bird and dinobirdMEANS

(dinosaur or bird) and dinobirdNOT

dinosaur or (bird and dinobird)

CQL features: parentheses

Parentheses may be used to override the defaultleft-to-right parsing of boolean operators.

● dinosaur and (bird or dinobird)● dinosaur or (bird and dinobird)● (bird or dinosaur) and (feathers or scales)● "feathered dinosaur" and (yixian or jehol)● (((a and b) or (c not d) not (e or f and g)) and h not i) or j

CQL features: pattern matching

There are two pattern-matching characters:* matches any number of characters? matches any single character

A preceding backslash removes their special meaning.

● dinosaur* – matches “dinosaurs”, “dinosauria”● *sauria – matches “dinosauria”, “carnosauria”● man?raptor – matches “maniraptor”, “manuraptor”● man?raptor* – matches the plurals of these● "the comp*saur" – matches “the complete dinosaur”● char\* – matches literal “char*”

CQL features: word anchoring

A word beginning with “^” must occur at the start of itsfield. A word ending with “^” must occur at the end ofits field.

● dinosaur – matches “the complete dinosaur”● dinosaur^ – also matches● ^dinosaur – does not match● the – matches “the complete dinosaur”● ^the – also matches● the^ – does not match

CQL features: indexes

A term of the form name=value is a query for the specifiedvalue occurring within the named index.

● title=Churchill – finds biographies of Churchill● author=Churchill – finds books written by him● title=dinosaur and author=farlow● title=(dinosaur and bird)● subject=(dinosaur* or pterosaur*)

Index names are case-insensitive, so “title” is the sameindex as “TITLE”, “Title” or “tiTLe”.

CQL features: prefixes

The meaning of an index can be specified more fullyby a prefix indicating what context set it is from. Themeaning of “title” is different in cross-domain searching(Dublin Core), bibliographic searching (Bath Profile)and heraldry.

● dc.title="the complete dinosaur"● property.title=freehold● heraldry.title=(viscount or duke)● cql.serverChoice=fruit● cql.resultSet=YXJjaGJpc2hvcAp

Prefixes are case-insensitive.

CQL features: context sets

A context set is a set of indexes that are related to aparticular area (plus some other more esoteric stuff thatyou can ignore).

For example, the Dublin Core context set containsindexes for searching against the fifteen DC elements:

title, creator, subject, description, publisher,contributor, date, type, format, identifier,source, language, relation, coverage, rights.

The context set prose must define their semantics.

CQL features: some context sets

A few core sets created by the SRW editorial board:● CQL – for core indexes such as resultSet● DC – for metadata searching with Dublin Core● Rec – metadata about the record, not the resource● Net – network concepts such as hostname and port

Also, many application-specific sets:● Bath, Zthes, CCG, Music● Rel – deep voodoo for relevance matching● GILS is in development

Where do context sets come from?● You can just make them up! No-one can stop you!

A digression on the CQL context set

The CQL context set is special. It contains some “magic”indexes:

● cql.anywhere – searches in all the indexes available

● cql.serverChoice – allows the server to choose whateverindex or indexes are suitable

● cql.resultSetId – finds the records obtained in a previoussearch, e.g. for refinement by combining with otherquery terms.

CQL features: relations

Usually “=” connects an index with its relation, but all theother obvious numeric relations are supported:

● Height = 13● numberOfWheels <= 3● numberOfPlates = 18● lengthOfFemur > 2.4● BioMass >= 100● NumberOfToes <> 3 (inequality)

CQL features: special relations

The keywords “any” and “all” can be used as relations,indicating that any one of, or all of, the words specifiedin the term must be found in the index:

● author all "kernighan ritchie"– shorthand for

author=kernighan and author=ritchie

● author any "kernighan ritchie thompson"– shorthand for

author=kernighan or author=ritchie orauthor=thompson

CQL features: esoterica

“You are not expected to understand this.”– comment in the Unix Version 7 source code.

The point is that new users are not required to understandthis, and may happily use CQL for many years – perhapsforever – without needing to.

CQL esoterica: proximity

The “prox” boolean, by default, requires its operandsto be next to each other, in either order:

● cervical prox vertebra– equivalent to

"cervical vertebra" or "vertebra cervical"

● (cervical or dorsal) prox vertebra– equivalent to

"cervical vertebra" or "dorsal vertebra" or"vertebra cervical" or "vertebra dorsal"

CQL esoterica: proximity II

Modifiers can generalise the semantics of proximity:

● cervical prox/distance<=5/ vertebrae– within five words of each other

● cervical prox/distance=0/unit=sentence vertebrae– within the same sentence

● cervical prox/distance>0/unit=paragraph vertebrae– in different paragraphs

● cervical prox/ordered vertebrae– in the specified order: exactly equivalent to

"cervical vertebra"

CQL esoterica: relation modifiers

Modifiers can refine the semantics of relations:

● title =/stem dig– finds “dig”, “digging”, “dug”, etc.

● title any/relevant "dinosaur bird reptile"– finds “sauropods”, “avian”, “crocodile”, “snake”, etc.

● author =/fuzzy tailor– finds “Mike Taylor”

● phoneNumber exact/fuzzy "020 8348 6768"– finds “020 8348 6769”

CQL esoterica: relation modifiers II

Relation modifiers can be overloaded to specify extrainformation about the term that the relation joins to theindex:

● createdDate >/isoDate "2004-03-12 09:45:00"– the term is in ISO 8601 format.

● Location within/geom.polygon "(12,46) (15,52)"– the term indicates a polygon of two points (i.e. a straight line) rather than the corners of a rectangle.

Modifiers can refine the semantics of boolean operators.We've already seen some examples of this in proximity.

● cervical prox/distance<=5/ vertebrae– within five words of each other

● cervical or/exclusive vertebrae– one or the other, but not both.

● "denenberg or/rel.mean "information retrieval"● "denenberg or/rel.sum "information retrieval"● "denenberg or/rel.max "information retrieval"

– average, total or maximum relevance of operands

CQL esoterica: boolean modifiers

So far, we have been free and easy with index prefixessuch as “dc”. But how do we know what they mean?

Why should “dc” mean Dublin Core rather than DeepCustard?● dc.custardDepth <= 20

Why should “bath” mean the Bath Profile for bibliographicsearching instead of plumbing supplies?● bath.capacityInGallons > 45

CQL esoterica: prefix mapping

Prefixes are just convenient, easy-to-type abbreviations.The real identifier of a context set is its URI.

For example, the Dublin Core context set isinfo:srw/cql-context-set/1/dc-v1.1

but we map that URI to a prefix for convenience.

This is exactly like XML namespaces: they are identifiedby URIs, but the URIs do not appear in the names ofelements or attributes: short prefixes are used instead.

CQL esoterica: prefix mapping II

In XML, a prefix is associated with a namespace using:● <element xmlns:prefix="http://example.org/xyz/">

In CQL, a prefix is associated with a namespace using:● >prefix=http://example.org/xyz/

and the rest of the query follows.

The following queries are exactly equivalent:● >dc=info:srw/cql-context-set/1/dc-v1.1 dc.title=fish● >yx=info:srw/cql-context-set/1/dc-v1.1 yx.title=fish

Most applications will have established default mappings.

CQL esoterica: prefix mapping III

It is possible to establish the context set from whichindexes with no explicit prefix are taken by omitting the“prefix=” part from the mapping:

● >http://example.org/heraldry/ title=baron and side=sinister

So the following queries are exactly equivalent:● >info:srw/cql-context-set/1/dc-v1.1 title=fish● >yx=info:srw/cql-context-set/1/dc-v1.1 yx.title=fish

CQL esoterica: prefix mapping IV

Finally ... Finally! :-)

Prefix mappings can be stacked up:

● >dc = info:srw/cql-context-set/1/dc-v1.1>bath=http://zing.z3950.org/cql/bath/2.0/>rec=info:srw/cql-context-set/2/rec-1.0

rec.created < 2004-10-09 anddc.title=ecology andbath.conferenceName=dinosaur

(Yes, this is all one query.)

CQL esoterica: prefix mapping V

Don't try this at home.

CQL esoterica: prefix mapping VI

Chapter 4: Applications

CQL has been deployed in many kinds of application:

● Google-like structureless searching● Simple metadata searching with the Dublin Core● Bath Profile for bibliographic data● Zthes profile for hierarchical thesaurus navigation● CCG for collectable card games● Music – musicalKey, arranger, duration, etc.● GILS (Global Information Locator Service)● ... your application goes here!

Chapter 5: Implementations

There are good-quality free CQL implementationsin several important languages:

● Java (Mike Taylor's CQL-Java package)● C/C++ (Adam Dickmeiss in Index Data's YAZ)● Python (Rob Sanderson in Cheshire)● Perl (Ed Summers' CQL::Parser module)● Visual Basic is in development (Thomas Habing)● ... your language goes here!

Conclusion: What to take home

● CQL makes easy queries easy and hard ones possible● You can use it well without learning the hard bits● It is used in SRW/SRU but also applicable elsewhere● It is extensible through context sets● Existing context sets support lots of applications● There are free implementations in several languages● Tutorial on-line at:

http://zing.z3950.org/cql/intro.html

CQL esoterica: relation modifiers II

Relation modifiers can be used to define essentially newrelations. Some hypothetical examples:

● location </geom.within "(12,46) (15,52)"– points within the specified rectangle

● task >/proj.prerequisite uiDesign– tasks that must be performed before the design of the user interface

● location =/geography.sameState "Las Vegas"– places in the same state as Las Vegas

CQL – a Common Query LanguageMike Taylor CQL – a Common Query Language 1. What CQL is 2. Motivation 3. Examples and explanation 4. Applications 5. Implementation.

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