An XML database is a data persistence software system that allows data to be specified, and sometimes stored, in XML format. These data can then be queried, transformed, exported and returned to a calling system. XML databases are a flavor of document-oriented databases which are in turn a category of NoSQL database (meaning Not (only) SQL).
Arguably, the rise of NoSQL as a far more important topic than XML Databases per se has eclipsed use of the term "XML Database." XML is now just one format to specify documents that can be stored in a variety of NoSQL databases. E.g. BaseX, eXist, MarkLogic, Oracle, DB2 and others store XML natively.[not in citation given]
Rationale for XML in databases
There are a number of reasons to directly specify data in XML or other document formats such as JSON. For XML in particular, they include:
- An enterprise may have a lot of XML in an existing standard format
- Data may need to be exposed or ingested as XML, so using another format such as relational forces double-modeling of the data
- XML is very well suited to sparse data, deeply nested data and mixed content (such as text with embedded markup tags)
- XML is human readable whereas relational tables require expertise to access
- Metadata is often available as XML
- Semantic web data is available as RDF/XML
O'Connell gives one reason for the use of XML in databases: the increasingly common use of XML for data transport, which has meant that "data is extracted from databases and put into XML documents and vice-versa".[needs update] It may prove more efficient (in terms of conversion costs) and easier to store the data in XML format. In content-based applications, the ability of the native XML database also minimizes the need for extraction or entry of metadata to support searching and navigation.
XML Enabled databases
XML enabled databases typically offer one or more of the following approaches to storing XML within the traditional relational structure:
- XML is stored into a CLOB (Character large object)
- XML is `shredded` into a series of Tables based on a Schema
- XML is stored into a native XML Type as defined by ISO Standard 9075-14
RDBMS that support the ISO XML Type are:
Typically an XML enabled database is best suited where the majority of data are non-XML. For datasets where the majority of data are XML, a native XML database is better suited.
Example of XML Type Query in IBM DB2 SQL
SELECT id, vol, xmlquery('$j/name', passing journal AS "j") AS name FROM journals WHERE xmlexists('$j[licence="CreativeCommons"]', passing journal AS "j")
Native XML databases
These databases are typically better when much of the data is in XML or other non-relational formats.
- BaseX http://basex.org/
- eXistDB http://exist-db.org/exist/apps/homepage/index.html
- MarkLogic http://marklogic.com
All the above databases uses XML as an interface to specify documents as tree structured data that may contain unstructured text, but on disk the data is stored as "optimized binary files." This makes query and retrieval faster. For MarkLogic it also allows XML and JSON to co-exist in one binary format.
Key features of native XML databases include:
- Has an XML document as at least one fundamental unit of (logical) storage, just as a relational database has a row in a table as a fundamental unit of (logical) storage.
- Need not have any particular underlying physical storage model. For example, NXDs can use optimized, proprietary storage formats. This is a key aspect of XML databases. Managing XML as large strings is inefficient due to the extra markup in XML. Compressing and indexing XML allows the illusion of directly accessing, querying and transforming XML while gaining the performance advantages of working with optimized binary tree structures.
In addition to XPath, XML databases support XSLT as a method of transforming documents or query-results retrieved from the database. XSLT provides a declarative language written using an XML grammar. It aims to define a set of XPath filters that can transform documents (in part or in whole) into other formats including plain text, XML, or HTML.
But big picture, XML persistence describes only one format in the larger, faster moving NoSQL movement at this time. Many databases support XML plus other formats, even if XML is internally stored as an optimized, high-performance format and is a first-class citizen within the database. (see Google Trends Link above to see relative popularity of terms).
|Name||License||Native Language||XQuery 3.0||XQuery Update||XQuery Full Text||EXPath Extensions||EXQuery Extensions||XSLT 2.0|
|eXist||LGPL License||Java||Partial||Proprietary[not in citation given]||Proprietary||No||Yes||Yes|
- "Google Trends of XML Database vs NoSQL".
- Nicola, Matthias (28 September 2010). "5 Reasons for Storing XML in a Database". Native XML Database. Retrieved 17 March 2015.
- Feldman, Damon (11 April 2013). Moving from Relational Modeling to XML and MarkLogic Data Models. MarkLogic World. Retrieved 17 March 2015.
- O'Connell, S. (2005). "Section 9.2". Advanced Databases Course Notes (Syllabus) (Southampton, England: University of Southampton).
- "XML Schema Storage and Query: Basic". Oracle XML DB Developer's Guide, 10g Release 2. Oracle Corporation. August 2005. Retrieved 17 March 2015.. Section Creating XMLType Tables and Columns Based on XML Schema
- "ISO/IEC 9075-14:2011: Information technology -- Database languages -- SQL -- Part 14: XML-Related Specifications (SQL/XML)". International Organization for Standardization. 2011. Retrieved 17 March 2015.
- "pureXML overview -- DB2 as an XML database". IBM Knowledge Center. IBM. Retrieved 17 March 2015.
- "Using XML in SQL Server". Microsoft Developer Network. Microsoft Corporation. Retrieved 17 March 2015.
- "XMLType Operations". Oracle XML DB Developer's Guide, 10g Release 2. Oracle Corporation. August 2005. Retrieved 17 March 2015.
- "8.13. XML Type". PostgreSQL 9.0.19 Documentation. Retrieved 17 March 2015.
- PostgreSQL - Data Types - XML Type
- Siegel, Erik; Retter, Adam (December 2014). "4. Architecture". eXist. O'Reilly & Associates. ISBN 978-1-4493-3710-0. Retrieved 18 March 2015.
- Kellogg, Dave (11 April 2010). "Yes, Virginia, MarkLogic is a NoSQL System". Kellblog. Retrieved 18 March 2015.
- "XQuery Update Extension". Retrieved 18 March 2015.
- XML Databases - The Business Case, Charles Foster, June 2008 - Talks about the current state of Databases and data persistence, how the current Relational Database model is starting to crack at the seams and gives an insight into a strong alternative for today's requirements.
- An XML-based Database of Molecular Pathways (2005-06-02) Speed / Performance comparisons of eXist, X-Hive, Sedna and Qizx/open
- XML Native Database Systems: Review of Sedna, Ozone, NeoCoreXMS 2006
- XML Data Stores: Emerging Practices
- Bhargava, P.; Rajamani, H.; Thaker, S.; Agarwal, A. (2005) XML Enabled Relational Databases, Texas, The University of Texas at Austin.
- Initiative for XML Databases
- XML and Databases, Ronald Bourret, September 2005
- The State of Native XML Databases, Elliotte Rusty Harold, August 13, 2007
- Qualcomm Qizx Official website