Editing XML schema (section)

{{short description|Description of a type of XML document}}
{{about|XML schemas in general|information on the W3C-recommendation|XML Schema (W3C)}}

An '''XML schema''' is a description of a type of [[Extensible Markup Language|XML]] document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself. These constraints are generally expressed using some combination of grammatical rules governing the order of elements, [[Boolean predicates]] that the content must satisfy, data types governing the content of elements and attributes, and more specialized rules such as [[uniqueness quantification|uniqueness]] and [[referential integrity]] constraints.

There are languages developed specifically to express XML schemas. The [[document type definition]] (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas. Two more expressive XML schema languages in widespread use are [[XML Schema (W3C)|XML Schema]] (with a capital ''S'') and [[RELAX NG]].

The mechanism for associating an XML document with a schema varies according to the schema language. The association may be achieved via markup within the XML document itself, or via some external means.

The XML Schema Definition is commonly referred to as XSD.