XML Schema and their Attributes

An XML Schema defines the elements, child elements and the attributes that can appear in a document. It can also define the order, the number of child elements, the data types for elements and attributes. In DBMS, Schema is a description of a database structure. Internal structures such as tables and fields can be defined using schemas. Schema defines tables and fields that make up the data. Schemas are defined using constraints.


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There are two types of constraints:

1. CONTENT CONSTRAINTS
2. DATATYPE CONSTRAINTS

An XML schema is a set of schema components in which there are 13 kinds of grouped components under 3 categories primary, secondary and helper components.

The primary components include

* SIMPLE TYPE DEFINITIONS
* COMPLEX TYPE DEFINITIONS
* ATTRIBUTE TYPE DEFINITIONS
*? ELEMENT DECLARATIONS

An XML schema consists of components such as type definitions and element declarations. They are used to access the validity of the well-formed element and attribute information items.
The purpose of XML schema structures schema describes a class of XML documents by using schemas components. Schemas provide specifications and additional information such as normalization and element values. Thus, XML schemas can be used to describe and catalogue XML documents. The XML schema validators allow us to check whether the instance of an document meets the requirements. The XML data can be described and validated using the XSD (XML Schema Editor). XSD is written in XML, so it doesn’t require a parser.

XML schemas are successors of the DTD’s. XML schemas are extensible, richer and more useful than DTD’s. XML schemas can support data types and name spaces. The difference between the schemas and DTD’s are:

DIFFERENCES

DTD'S
SCHEMAS
LANGUAGE DTD's are written using EBNF (Extended Backus-Naur Form) Schemas are written using XML.
DATA CONSTRAINTS DTD's have minimal data constraints. Schemas allow more specific constraints
USER DEFINED TYPES DTD's limit to a fixed set of content models Schemas provide greater flexibility and expressing content.



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