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What is a database schema?


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2008-03-16 21:40:34
2008-03-16 21:40:34

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

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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.

The Star & Snowflake are two common type of database Schema.

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

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.

A database schema is an actual plan for a database system. Sub-schemas are separate divisions that are used to achieve the plan for the database system.

In the manner described in the database schema.

The database schema is the description of the database. It defines how the database is set up and what data it is to contain and control. This description is given to the DBMS to manage the data. The database state is an image of how the database looks at any given time. The database schema is set unless you change how the database is structured. The database state will change whenever new data is entered into the database and the database is updated.

In DBMS,Schema is the overall Design of the Database.Instance is the information stored in the Database at a particular moment.In programming,you declare a variable which corresponds to "Schema".But its values changes as and when required which corresponds to "Instance". Google about levels of Database Abstraction. Physical Schema describes database design at physical level while a logical schema describes the database design at the logical level.A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.

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: 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.

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.

The properties of a database are commonly called a schema. The schema describes the tables in the database, the fields inside each table, the contents of each field, etc.

A Database has three views. The External Schema: What the end user sees. The Internal Schema: What the programmers of the program see. The Conceptual Schema: The basic plan of the database. Most of the time this is in paper form as a Conceptual Schema Diagram (CSD)

Schema name or database qualifier indicates the ownership of the object. Schema name for a database object can be found using below sql -SELECT owner FROM all_objects WHERE object_name = 'tablename';

overall design of the database

The first and most important step in designing a database is creating a schema. After creating schema tables are created and data is entered in to the tables.

In DB2 Schema is not a user, its is just a space to store database objects,

Constraints are part of a database schema definition.They define certain properties that data in a database must comply with.

In non technical terms... 1. the database schema sets what data you can store in a data base and how it should be stored. (The plans for the house.) 2. the database state is the actual database complete with all its data. (The house after it has been built and the family has moved in!)

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 a relational database, the schema defines the tables, the fields in each table, and the relationships between fields and tables.

the schema can be termed either conceptual or logical or physical. But mostly speaking, the term schema is used to refer to a logical structure.

A database can be created by first creating a schema then creating tables. The tables are linked to each other.

* The database schema * Schema objects * Indexes * Tables * Fields and columns * Records and rows * Keys * Relationships * Data types * just follow this link

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