An XML database is a data persistence software system that allows data to be specified, and sometimes stored, in XML format. This data can 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. 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 Provides a solution for Object-relational impedance mismatch Steve 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". 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: IBM DB2 (pureXML) Microsoft SQL Server Oracle Database PostgreSQL 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 Native XML databases are especially tailored for working with XML data. As managing XML as large strings would