Database (DBMS) Languages

ashish — Thu, 06 Aug 2009 06:36:35 +0000

Once the design of the database is completed and a DBMS is chosen to implement the database, the next step is to define the conceptual and internal schema for the data and any mappings between the two. The languages that are used to do so are :

• Data Definition Language (DDL): To specify the conceptual schema of a database by the DBA and by database designers. In many DBMSs, the DDL is also used to define internal and external schemas (views).
• In some DBMSs, separate storage definition language (SDL) are used to define internal and external schemas.
The mappings between the two schemas may be specified in either one of these languages. For a true three-schema architecture, a third language is needed, the view definition language (VDL), to specify user views and their mappings to conceptual schema.

Once the database schemas are compiled and the database is populated with data, users must have some means to manipulate the data.
• Data Manipulation Language (DML): Used to specify database retrievals and updates.
• DML commands (data sublanguage) can be embedded in a general-purpose programming language (hostlanguage), such as COBOL, C or an Assembly Language.
• Alternatively, stand-alone DML commands can be applied directly (query language).
DMLs can be high level(set-oriented, nonprocedural) or low level)record-oriented, procedural). A high-level DML can be embedded in a host programming language, or it can be used as a stand-alone language; in the latter case it is often called as a query language.

In current DBMSs, a comprehensive integrated language is used that includes constructs for conceptual schema definition, view definition, and data manipulation. A typical example of a comprehensive database language is the SQL relational database language.
Whenever DML commands are embedded in a general-purpose programming language, that language is called the host language and the DML is called the data sublanguage.

Database System Concepts – Data Model, Schemas and Database state

ashish — Tue, 04 Aug 2009 03:05:29 +0000

A data model is a collection of concepts that can be used to describe the structure of a database. By structure of the database we mean the data types, relationships, and constraints that should hold on the data. Most data models also include a set of basic operations for specifying retrievals and updates on database.

Categories of Data Models:
- High level or Conceptual data models : These models provide concepts that are close to the way many users perceive data. They use concepts such as entities, attributes, and relationships. An entity represents a real-world object or concept such as an employee or a project. An attribute represents property of interest that describes an entity such as employee’s salary or name. A relationship represents an interaction among the entities.
- Representational data models : These models provides concept that may be understood by end users but that are not too far removed from the way data is organized within the computer. They are used most frequently in traditional commercial DBMSs and they include the widely used relational model as well as the network and hierarchical models. These models represent data by using record structures and hence are sometimes called record-based data models.
- Low level or Physical data models : These models provide concepts that describe the details of how the data is stored in the computer by representing information such as record formats, record orderings, and access paths. An access path is a structure that makes the search for particular database records efficient.

Schemas:
The description of a database in any data model is called the database schema which is specified during the database design and is not expected to change frequently. A displayed is called a schema diagram.

A schema diagram displays only some aspects of a schema, such as names of record types and data items, and some types of constraints.

Database State or Iinstance: The actual data in a database changes every time data is inserted, deleted, or modified. The data in the database at a particular moment in time is called a database state or a snapshot. It is also called the current set of occurrences or instances in the database.

Distinguish between Database State and Database Schema:
When a new database is defined, we specify its database schema only to the DBMS. At this point, the corresponding database state is empty state. The initial state of the database is got 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.

Learn Software Development » Schemas

Database (DBMS) Languages

Database System Concepts – Data Model, Schemas and Database state