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  Types of database systems on the basis of site location :-  1. Centralized database system :-  The centralized database system consists of a single processor together with it's associated data storage devices and other peripherals. It is physically confined to a single location. Data can be accessed from the multiple sites with the use of a computer network while the database is maintained at the central site.  Disadvantages of centralized database system :-  When the central site computer or database system goes down, then every one is blocked from using the system until the system comes back.  Communication costs from the terminals to the central site can be expensive.  2. Parallel database system :-  Parallel database system architecture consists of a multiple central processing units and data storage disk parallel. Hence, they improve processing and Input/Output speeds. Parallel database systems are used in the application that have to query...

  Types of database system :- The DBMS can be classified according to the number of users and the database site locations. These are :-  On the basis of the number of users :-  Single-user DBMS Multi-user DBMS      2. On the basis of the site location :-  Centralized DBMS Parallel DBMS Distributed DBMS Client/server DBMS In this section, we will discuss about some of the important types of DBMS system, which are presently being used. The database system may be multi-user or single-user. The configuration of the hardware and the size of the organization will determine whether it is a multi-user system or single user system.  In single user system the database resides on one computer and is only accessed by one user at a time. This one user may design, maintain and write database programs.  Due to large amount of data management most systems are multi-user. In this situation the data are both integrated and shared. A database is integrated when...

  Functions and service of DBMS :-  A DBMS performs several important functions that guarantee integrity and consistency of data in the database. Most of these functions are transparent to end users. There are the following important functions and services provided by a DBMS. Data storage management :- It provides a mechanism for management of permanent storage of the data. The internal schema defines how the data should be stored by the storage management mechanism and the storage manager interfaces with the operating system to access the physical storage.  Data Manipulation management :- A DBMS furnishes users with the ability to retrieve, update and delete existing data in the database.  Data Definition services :- The DBMS accepts the data definitions such as external schema, the conceptual schema, the internal schema and all the associated mapping in source form.  Data dictionary/system catalog management :- The DBMS provides a data dictionary or system cat...

  Structure and components of a DBMS :-  A typical structure of a DBMS with it's components and relationship between them. The DBMS software is partitioned into several modules. Each module or component is assigned a specific operations to perform. Some of the functions of the DBMS are supported by operating systems to provide basic services and DBMS is built on top of it. The physical data and system catalog are stored on a physical disk. Access to the disk is controlled primarily by OS, which schedules disk input/output. Therefore, while designing a DBMS it's interface with the OS must be taken into account.  Components of a DBMS:-  The DBMS accepts the SQL commands generated from a variety of user interface,produces query evaluation plans,executes these plans against the database and returns the answers. Major software modules or components of DBMS are as follows :-  1). Query processor :- The query processor transform user queries into a series of low level ...

  Database languages :-  A DBMS must provide appropriate languages and interfaces for each category of users to express database queries and updates. Database languages are used to create and maintain database on computer. There are large numbers of languages like Oracle, MySQL, MS access, dBase,FoxPro etc. SQL statements commonly used in Oracle and MS access can be categorized as Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL) and Transaction Control Language (TCL).  Data Definition Language :-  It is a language that allows the users to define data and their relationship to other types of data. It is mainly used to create , alter, drop, rename the files, databases, data dictionary and tables within databases.  It is also used to specify the structure of each table, set of associated values with each attributes, integrity constraints, security and authorization information for each table and physical storage structur...

The procedure for Database access :-  Any access to the stored data by the data manager. The steps involved in database access can be summarised as shown in figure. A users request for data is received by the data manager, which determines the physical record required. The decision as to which physical record is needed may require some preliminary consultation of the database and/or the data dictionary prior to the access of the actual data itself.  The data manager sends the request for a specific physical record to the file manager. The file manager decides which physical block of secondary storage devices contains the required record and send the request for the appropriate block to the disk manager. A block is a unit of physical input/output operations between primary and secondary storage. The disk manager retrieve the block and sends it to the file manager, which sends the required record to the data manager.  What happens when user issues a request to DBMS?  T...