Database schema（From Wikipedia）

A database schema (pronounced skee-ma, /ˈski.mə/) of a database system is its structure described in a formal language

 supported by the database management system (DBMS) and refers to the organization of data to create a blueprint of how

 a database will be constructed (divided into database tables). The formal definition of database schema is a set of formulas

 (sentences) called integrity constraints imposed on a database. These integrity constraints ensure compatibility between

 parts of the schema. All constraints are expressible in the same language. A database can be considered a structure in 

realization of thedatabase language.[1] The states of a created conceptual schema are transformed into an explicit mapping,

 the database schema. This describes how real world entities are modeled in the database.

"A database schema specifies, based on the database administrator's knowledge of possible applications, the facts that

 can enter the database, or those of interest to the possible end-users."[2] The notion of a database schema plays the same

 role as the notion of theory in predicate calculus. A model of this “theory” closely corresponds to a database, which can be

 seen at any instant of time as a mathematical object. Thus a schema can contain formulas representing integrity constraints 

specifically for an application and the constraints specifically for a type of database, all expressed in the same database

 language.[1] In a relational database, the schema defines the tables, fields, relationships, views, indexes,packages, 

procedures, functions, queues, triggers, types, sequences, materialized views, synonyms, database links, directories, 

Java, XML schemas, and other elements.

Schemata are generally stored in a data dictionary. Although a schema is defined in text database language, the term is

 often used to refer to a graphical depiction of the database structure. In other words, schema is the structure of the database

 that defines the objects in the database.

In an Oracle Database system, the term "schema" has a slightly different connotation. For the interpretation used in an Oracle

 Database, seeschema object.

Levels of database schema

• Conceptual schema, a map of concepts and their relationships.
• Logical schema, a map of entities and their attributes and relations
• Physical schema, a particular implementation of a logical schema
• Schema object, Oracle database object

Ideal requirements for schema integration

Completeness

All information in the source data should be included in the database schema.^[3]

Overlap preservation

Each of the overlapping elements specified in the input mapping is also in a database schema relation.^[3]

Extended overlap preservation

Source-specific elements that are associated with a source’s overlapping elements are passed through to the database schema.^[3]

Normalization

Main article: Database normalization

Independent entities and relationships in the source data should not be grouped together in the same relation in the database schema. 

In particular, source specific schema elements should not be grouped with overlapping schema elements, if the grouping co-locates

 independent entities or relationships.[3]

Minimality

If any elements of the database schema are dropped then the database schema is not ideal.[3]

These requirements influence the detailed structure of schemas that are produced. Certain applications will not require that all

 of these conditions are met, but these five requirements are the most ideal.

Example of two schema integrations

Example: Suppose we want a mediated (database) schema to integrate two travel databases, Go-travel and Ok-travel.

Go-travel has three relations:

Go-flight(f-num, time, meal)
Go-price(f-num, date, price)
Go-airline(airline, phone)

The attribute f-num is the flight number and meal is a boolean. The other attributes are self-explanatory.

Ok-travel has just one relation:

Ok-flight(f-num, date, time, price, nonstop)

'nonstop' is a boolean.

The overlapping information in Ok-travel’s and Go-travel’s schemas could be represented in a mediated schema:

Flight(f-num, date, time, price)[3]

See also

• Core Architecture Data Model
• Data Definition Language
• Data Design
• Data dictionary
• Data model
• Data modeling
• Data mapping
• Database
• Database integrity
• entity-relationship model
• Knowledge Representation
• Object-Role Modeling
• Relational algebra
• Schema Matching
• SQL
• Three schema approach

References

External links

• http://www.databaseanswers.org/data_models/
• http://weblogs.asp.net/scottgu/archive/2006/07/12/Tip_2F00_Trick_3A00_-Online-Database-Schema-Samples-Library.aspx
• http://msdn.microsoft.com/en-us/library/bb187299%28SQL.80%29.aspx
• http://www.ciobriefings.com/Publications/WhitePapers/DesigningtheStarSchemaDatabase/tabid/101/Default.aspx