In DBMSs where a clear separation is maintained between the conceptual and internal levels, the DDL is used to specify the conceptual schema only. Another language, the storage definition language (SDL), is used to specify the internal schema. The mappings between the two schemas may be specified in either one of these languages. For a true three-schema architecture, we would need a third language, the view definition language (VDL), to specify user views and their mappings to the conceptual schema, but in most DBMSs the DDL is used to define both conceptual and external schemas. Once the database schemas are compiled and the database is populated with data, users must have some means to manipulate the database. Typical manipulations include retrieval, insertion, deletion, and modification of the data. The DBMS provides a data manipulation language (DML) for these purposes. In current DBMSs, the preceding types of languages are usually not considered distinct languages; rather, a comprehensive integrated language is used that includes constructs for conceptual schema definition, view definition, and data manipulation. Storage definition is typically kept separate, since it is used for defining physical storage structures to fine-tune the performance of the database system, and it is usually utilized by the DBA staff. A typical example of a comprehensive database language is the SQL relational database language , which represents a combination of DDL, VDL, and DML, as well as statements for constraint specification and schema evolution. The SDL was a component in earlier versions of SQL but has been removed from the language to keep it at the conceptual and external levels only. There are two main types of DMLs. A high-level or nonprocedural DML can be used on its own to specify complex database operations in a concise manner. Many DBMSs allow high-level DML statements either to be entered interactively from a terminal (or monitor) or to be embedded in a general-purpose programming language. In the latter case, DML statements must be identified within the program so that they can be extracted by a pre-compiler and processed by the DBMS. A low-level or procedural DML must be embedded in a general-purpose programming language. This type of DML typically retrieves individual records or objects from the database and processes each separately. Hence, it needs to use programming language constructs, such as looping, to retrieve and process each record from a set of records. Low-level DMLs are also called record-at-a-time DMLs because of this property. High-level DMLs, such as SQL, can specify and retrieve many records in a single DML statement and are hence called set-at-a-time or set-oriented DMLs. A query in a high-level DML often specifies which data to retrieve rather than how to retrieve it; hence, such languages are also called declarative. Whenever DML commands, whether high-level or low-level, are embedded in a general-purpose programming language, that language is called the host language and the DML is called the data sublanguage . On the other hand, a high-level DML used in a stand-alone interactive manner is called a query language. In general, both retrieval and update commands of a high-level DML may be used interactively and are hence considered part of the query language . Casual end users typically use a high-level query language to specify their requests, whereas programmers use the DML in its embedded form. For naive and parametric users, there usually are user-friendly interfaces for interacting with the database; these can also be used by casual users or others who do not want to learn the details of a high-level query language. We discuss these types of interfaces next.
Once the design of a database is completed and a DBMS is chosen to implement the database, the first order of the day is to specify conceptual and internal schemas for the database and any mappings between the two. In many DBMSs where no strict separation of levels is maintained, one language, called the data definition language (DDL), is used by the DBA and by database designers to define both schemas. The DBMS will have a DDL compiler whose function is to process DDL statements in order to identify descriptions of the schema constructs and to store the schema description in the DBMS catalog.
Hello friends.. It is my first post on Database management system.my blog is dedicated to database management for the people who want to know the basics of database and In this topic I m going to cover the advantages of DBMS in brief. abstract view of the data to insulate application code from such details. 2)Efficient Data access:- 3) Data Integrity and Security:- If data is always accessed through the DBMS, the DBMS can enforce integrity constraints on the data. For example, before inserting salary information for an employee, the DBMS can check that the department budget is not exceeded. Also, the DBMS can enforce access controls that govern what data is visible to di_erent classes of users. 4)Data Administration:- When several users share the data, centralizing the administration of data can o_er signi_cant improvements. Experienced professionals who understand the nature of the data being managed, and how di_erent groups of users use it, can be responsible for organizing the data representation to minimize redundancy and for fine-tuning the storage of the data to make retrieval efficient. 5) Concurrent Access and crash recovery :- A DBMS schedules concurrent accesses to the data in such a manner that users can think of the data as being accessed by only one user at a time. Further, the DBMS protects users from the effects of system failures. 6) Reducing Application Development time:- in the DBMS. This, in conjunction with the high-level interface to the data, facilitates quick development of applications. Such applications are also likely to be more robust than applications developed from scratch because many important tasks are handled by the DBMS instead of being implemented by the application. 7) Permitting Interencing and Actions Using Rules :- Some database systems provide capabilities for defining deduction rules for inferencing new information from the stored database facts. Such systems are called deductive database systems. For example, there may be complex rules in the miniworld application for determining when a student is on probation. These can be specified declaratively as rules, which when compiled and maintained by the DBMS can determine all students on probation. In a traditional DBMS, an explicit procedural program code would have to be written to support such applications. But if the miniworld rules change, it is generally more convenient to change the declared deduction rules than to recode procedural programs. More powerful functionality is provided by active database systems, which provide active rules that can automatically initiate actions. 8)Providing Multiple Users Interfaces :- Because many types of users with varying levels of technical knowledge use a database, a DBMS should provide a variety of user interfaces. These include query languages for casual users; programming language interfaces for application programmers; forms and command codes for parametric users; and menu-driven interfaces and natural language interfaces for stand-alone users. Both forms-style interfaces and menu-driven interfaces are commonly known as graphical user interfaces (GUIs). Many specialized languages and environments exist for specifying GUIs. Capabilities for providing World Wide Web access to a database—or web-enabling a database—are also becoming increasingly common. 9)Representing Complex Relationships Among Data :- A database may include numerous varieties of data that are interrelated in many ways. Consider the example shown in Figure 01.02. The record for Brown in the student file is related to four records in the GRADE_REPORT file. Similarly, each section record is related to one course record as well as to a number of GRADE_REPORT records—one for each student who completed that section. A DBMS must have the capability to represent a variety of complex relationships among the data as well as to retrieve and update related data easily and efficiently. 10)Providing Backup And Recovery:- A DBMS must provide facilities for recovering from hardware or software failures. The backup and recovery subsystem of the DBMS is responsible for recovery. For example, if the computer system fails in the middle of a complex update program, the recovery subsystem is responsible for making sure that the database is restored to the state it was in before the program started executing. Alternatively, the recovery subsystem could ensure that the program is resumed from the point at which it was interrupted so that its full effect is recorded in the database.
Application programs should be as independent as possible from details of data representation and storage. The DBMS can provide an
A DBMS provides technique to store the data and received data more efficiently .This technique is more useful when we are accessing data from the external device.
Clearly, the DBMS supports many important functions that are common to many applications accessing data stored
