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From Everything.Sucks

A DTD defines the valid building blocks of an XML document. It defines the document structure with a list of validated elements and attributes. A DTD can be declared inline inside an XML document, or as an external reference. XML uses a subset of SGML DTD. As of 2009, newer XML namespace-aware schema languages (such as W3C XML Schema and ISO RELAX NG) have largely superseded DTDs. A namespace-aware version of DTDs is being developed as Part 9 of ISO DSDL. DTDs persist in applications that need special publishing characters, such as the XML and HTML Character Entity References, which derive from larger sets defined as part of the ISO SGML standard effort.

According to SIENCE DIRECT website DTD, as the first XML schema format, is very commonly used in practice. It is relatively simple to understand and concise (if syntactically awkward), it is supported by virtually every XML parser in existence, and it is sufficient for most document structural specifications. Unfortunately, it has many limitations for data interchange. One cannot directly map the database concepts of key and foreign key to ID and IDREFS (there is no support for compound keys, and the value of a key must be globally unique within a document, even to the exclusion of ID-typed attributes with other names). The concept of null values does not map to any aspect of DTD-based XML, meaning that relational database export is awkward. Primitive datatypes such as integers and dates are not possible to specify. All of these limitations, as well as a variety of other desiderata, led to the development of a newer specification in the early 2000s, called XML Schema.


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