Requirements and Desiderata for an XML Query Language

November 12, 1998

Context

We are coming to this primarily from the perspective of database management applications, E-commerce messages, and (especially) metadata management for DB applications.

It is our view that XML will be extensively used to encode DB queries and query results, i.e., fragments of databases. Hence, we are seeking a query language with expressiveness at least equal to conventional DB query facilities, e.g., SQL or OQL.

We believe in the importance of metadata for understanding data and building plug-and-play applications hence we are seeking a query language that can query both/either data and metadata encoded in either XML and/or RDF.

Requirements and Desiderata

• Ability to query multiple documents - We view this as essential for our applications.
• Integrated treatment of links - In some proposals, e.g., early XSL proposals, links are not treated as full rank mechanisms. We would like to see as much homogeniety as feasible in the treatment of elements and links..
• Query language for both XML and RDF - preferably as similar as possible. We need to be able to query both data and metadata.
• High order query language - We seek the ability to have variables in query language range over schema elements. (Note: SQL does not support this.) This facilitates the development of tools (e.g., mediators) which dynamically bind to schemas.
• Recursive queries - We would like to be able to specify regular expressions on paths (i.e., transitive closure queries).
• Uniform treatment of attributes and elements. XML has two different syntaxes which are often used equivalently: attributes specifyied in the tags, and nested elements. A number of XML dialect proposals treat these as equivalent. We would therefore like a query language which also did so. Obviously, attributes can not be nested.
• Typed query language and extensible type algebra. We favor strong typing of the query language and an extensible type algebra (for consistency/conversion of types) - see discussion below on measurement units/dimensionality.
• Measurement units/dimensionality types enforced/automatically converted in query language.
• Ability to query schemas, as well as data. Ability to query metadata repositories.
• Coordinate query language efforts with schema efforts - A number of persons have noted that the query language and schema specification efforts should be closely coordinated. We concur.
• Query multi-dimensional tables. Many web/XML documents contain multi-dimensional tables. Tree-based schema/query language proposals do not fully capture this structure, hence make it very clumsy to query.