A database schema (/ˈski.mə/ SKEE-ma) 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 as a blueprint of how a database is constructed (divided into database tables in case of Relational Databases). 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 the database language. 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." 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. 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, XML schemas, and other elements.
Schemas 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. In the context of Oracle databases, a schema object is a logical data storage structure. In an Oracle database, associated with each database user is a schema. A schema comprises a collection of schema objects. Examples of schema objects include:
- database links
On the other hand, non-schema objects may include: users, roles, contexts, directory objects.
Schema objects do not have a one-to-one correspondence to physical files on disk that store their information. However, Oracle databases store schema objects logically within a tablespace of the database. The data of each object is physically contained in one or more of the tablespace's- datafiles. For some objects (such as tables, indexes, and clusters) a database administrator can specify how much disk space the Oracle RDBMS allocates for the object within the table-space's datafiles.
There is no necessary relationship between schemas and table-spaces: a tables-pace can contain objects from different schemas, and the objects for a single schema can reside in different table-spaces.
Returning to non-Oracle databases, the levels of database schema are:
- 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
Ideal requirements for schema integration
All information in the source data should be included in the database schema.
Each of the overlapping elements specified in the input mapping is also in a database schema relation. This contrasts with extended overlap preservation, which is are source-specific elements that are associated with a source’s overlapping elements are passed through to the database schema.
If any elements of the database schema are dropped then the database schema is not ideal.
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)
f-num is the flight number and meal is a boolean.
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)