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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