QueryLanguages

ProLearn Query Language Definition

Welcome to the ProLearn Query Language Initiative!

For the 0.7 stable version of the document please look here: [1].

For the 0.8 version of the document please look here: [2]

For the 1.0 stable version of the document please look here: [3]

If you want to leave comments and/or questions please use this page: [4].

For a summary of all the developed prototypes please look at: [5].

Thanks for all the help!

Introduction

This report describes the first definition of the ProLearn Query Language (PLQL), a “Query language for learning repositories” defined in the context of the ProLearn NOE (WP4). Our background is SQI, a query transport standard (also grown out of ProLearn, but now a CENN ISSS standard) that is becoming widely used within the e-learning community. SQI’s distinguishing feature is to be (on purpose) agnostic about query language; as such, it can be used together with any query language. Going beyond SQI requires adding support to a specific query language, designed with the specific objective of supporting queries to learning object repositories.

PLQL is primarily a “query interchange format” used by e-learning application developers for querying the repositories from inside their programs, using the SQI protocol; it is up to them to define a user interface, normally based upon simple keyword-based forms; it is up to the repository managers to build the adapters that will be compliant with PLQL levels.

PLQL combines two features: support of keyword-based search and support of exact matching. The former is required for extracting LOs which best match informal descriptions, typically provided through search interfaces; for this feature we borrow concepts from CQL, a well-established language used for library search. The latter is required when the application knows the metadata schema, which is typically in hierarchical format, and therefore can extract data which match specific items of the repository schema. Given the wide use of XML-based metadata schemas and their typical hierarchical structure (e.g. LOM), we have borrowed concepts from XPATH for this purpose.

The language should then be capable of performing exact search, when the structure of the metadata is known, as well as ranked retrieval, when the structure of the metadata is not known; perhaps the most challenging aspect of the language's semantics is to define the query semantics when both query aspects should be taken into consideration.

In designing PLQL, we aim at supporting also repositories with a minimum level of support. Thus, one of the main concerns of the language design is on providing progressive layers, supporting increasingly expressive power, so that even simple repositories can support the low layers.