Database schema are the logical structure of entities (tables or object classes) and their relationships (keys, or object associations) in a database. Schema can exist at different levels of abstraction (see: en.wikipedia.org/wiki/Database_schema). Schema themselves may or may not exist as such in a database. In general schema at the higher levels of abstraction are design models that are captured in printed documents, and authored by business analysts and database designers, rather than created by database programmers and stored in the database itself. But in some types of databases, schema at any level of abstaction can be realized physically. In Oracle databases, schema can be realized as schema objects, which are a part of an Oracle database. These schema objects may represent a human user's conceptual model of the knowledge captured in an enterprise database. Thus defined, there can be many schema that can be associated with one physical database. In other kinds of relational database, an abstract user's schema can be represented by logically linked metadata, views, and stored procedures assocated with a user class. In object-oriented databases, especially those based on the highly self-referential language Smalltalk (e.g. Gemstone/S), schema are realized as "physical" objects in the database as a matter of course, as well as in the logical models captured in design documents. The schema objects in such databases are, in essence, the classes comprising the infrastructure of the database or application. Oracle-style user schema are readily created; these would be realized as user interface orchestrator classes.
As I understand it, a database schema is a physical entity, it describes the structure of exactly how the data is stored and is itself stored by DBMS for reference. Data model, on the other hand, is an abstract representation of database.
A catalog is simple a group of related schema collected together in a defined namespace. While schema is a group of one or more related object collected in a common namespace.
difference between relation sehema and relation instance in dbms
Database Instance The term instance is typically used to describe a complete database environment, including the RDBMS software, table structure, stored procedures and other functionality. It is most commonly used when administrators describe multiple instances of the same database.Also Known As: environment Examples: An organization with an employees database might have three different instances: production (used to contain live data), pre-production (used to test new functionality prior to release into production) and development (used by database developers to create new functionality). RELATION SCHEMA A relation schema can be thought of as the basic information describing a table or relation. This includes a set of column names, the data types associated with each column, and the name associated with the entire table. For example, a relation schema for the relation called Students could be expressed using the following representation: Students(sid: string, name: string, login: string, age: integer, gpa: real) There are five fields or columns, with names and types as shown above.
Update and Alter are two SQL (Structured Query Language) commands used for modifying databases. Update statement is used to update existing records in a database. Update is a Data Manipulation Language (DML) statement. Alter SQL command is used to modify, delete or add a column to an existing table in a database. Alter is a Data Definition Language (DDL) statement.Commands that are used to define the structure of a database (database schema) are called DDL statements.
As I understand it, a database schema is a physical entity, it describes the structure of exactly how the data is stored and is itself stored by DBMS for reference. Data model, on the other hand, is an abstract representation of database.
In DB2, a user is an individual who is granted permissions to access and interact with the database, while a schema is a named collection of tables, views, procedures, and other database objects owned by a user. Users are associated with schemas to define their default schema for objects they create.
A database schema is described in a formal language supported by the database management system (DBMS). In a relational database, the schema defines the tables, the fields in each table, and the relationships between fields and tables. 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 Levels of database schema 1. Conceptual schema, a map of concepts and their relationships 2. Logical schema, a map of entities and their attributes and relations 3. Physical schema, a particular implementation of a logical schema 4. Schema object, Oracle database object 5. Schema is the over all structure of the database
The distinction between database schema and database state is very important. When we define a new database, we specify its database schema only to the DBMS. At this point, the corresponding database state is the empty state with no data. We get the initial state of the database when the database is first populated or loaded with the initial data. From then on, every time an update operation is applied to the database, we get another database state. At any point in time, the database has a current state. The DBMS is partly responsible for ensuring that every state of the database is a valid state-that is, a state that satisfies the structure and constraints specified in the schema. The DBMS stores the descriptions of the schema constructs and constraints-also called the meta-data-in the DBMS catalog so that DBMS software can refer to the schema whenever it needs to. The schema is sometimes called the intension, and a database state an extension of the schema.
We know that schema in the database is refer to the basic design of that database and is stored in an object . That object is known as schema object of database.
A catalog is simple a group of related schema collected together in a defined namespace. While schema is a group of one or more related object collected in a common namespace.
Logical Structure of database is know as database schema. All database entities are defined and relation between then is represented in structured way. It includes all constraints to be apply on each identity.
A data dictionary is a collection of metadata about the data elements in a database, where as, a database schema is a design blueprint for how data is structured and organized within a database. The data dictionary describes the data, while the schema defines how the data is stored.
The Star & Snowflake are two common type of database Schema.
In a relational database, the schema defines the tables, the fields in each table, and the relationships between fields and tables.
One can easily read a database schema through any software available currently. Simpy because a database schema is a blueprint which can be read by a software.
tablespace,segment,extent,oracle blocks.. are the logical schema of a database