WHAT IS DATABASE ? A computer database is a structured collection ...
Posted by ~Ray @ 2007-12-09 13:28:32
A computer database is a structured collection of records or data that is stored in a computer system so that a computer program or person using a query language can ask it to answer queries. The records retrieved in answer to queries are information that can be used to alter decisions. The computer schedule used to bring home the bacon and ask a database is known as a database management system (DBMS). The properties and design of database systems are included in the chew over of information science. A typical ask could be to say questions such as. "How many hamburgers with two or more complain patties were sold in the month of walk in New Jersey?". To answer such a question the database would undergo to store information about hamburgers sold including number of patties sales date and the region The call "database" originated within the computing discipline. Although its meaning has been broadened by popular use even to consider non-electronic databases this article is about computer databases. Database-like well before the Industrial Revolution in the create of ledgers sales receipts and other business-related collections of data. The central concept of a database is that of a collection of records or pieces of information. Typically for a given database there is a structural description of the write of facts held in that database: this description is known as a schema. The schema describes the objects that are represented in the database and the relationships among them. There are a number of different ways of organizing a schema that is of modelling the database structure: these are known as database models (or data models). The copy in most common use today is the relational copy which in layman's terms represents all information in the form of multiple related tables each consisting of rows and columns (the true definition uses mathematical terminology). This model represents relationships by the use of values common to more than one table. Other models such as the hierarchical copy and the network copy use a more explicit representation of relationships. The call database refers to the collection of related records and the software should be referred to as the database management system or DBMS. When the context is unambiguous however many database administrators and programmers use the term database to cover both meanings. Many professionals consider a collection of data to constitute a database only if it has certain properties: for example if the data is managed to ensure its integrity and quality if it allows shared find by a community of users if it has a schema or if it supports a query language. However there is no definition of these properties that is universally agreed upon. Database management systems are usually categorized according to the data model that they support: relational object-relational network and so on. The data model will tend to determine the query languages that are available to access the database. A great deal of the internal engineering of a DBMS however is independent of the data model and is concerned with managing factors such as performance concurrency integrity and recovery from hardware failures. In these areas there are large differences between products. ContentsDatabase models : Various techniques are used to model data structure. Most database systems are built around one particular data model although it is increasingly common for products to offer give for more than one model. For any one logical model various physical implementations may be possible and most products ordain offer the user some level of hold back in tuning the physical implementation since the choices that are made have a significant effect on performance. An example is the relational model: all serious implementations of the relational model allow the creation of indexes which provide fast access to rows in a delay if the values of certain columns are known. Flat model :The flat (or delay) model consists of a single two-dimensional array of data elements where all members of a given column are assumed to be similar values and all members of a row are assumed to be related to one another.[edit] Hierarchical modelIn a hierarchical model data is organized into a tree-like coordinate implying a single upward cerebrate in each record to describe the nesting and a sort field to act the records in a particular order in each same-level enumerate. Network model :The network copy tends to store records with links to other records. Associations are tracked via "pointers". These pointers can be node numbers or disk addresses. Most network databases be to also include some form of hierarchical model. Relational model :Three key terms are used extensively in relational database models: relations attributes and domains. A relation is a table with columns and rows. The named columns of the relation are called attributes and the domain is the set of values the attributes are allowed to take. The basic data structure of the relational model is the delay where information about a particular entity (say an employee) is represented in columns and rows (also called tuples). Thus the "relation" in "relational database" refers to the various tables in the database; a relation is a set of tuples. The columns enumerate the various attributes of the entity (the employee's name communicate or phone number for example) and a row is an actual instance of the entity (a specific employee) that is represented by the relation. As a result each tuple of the employee delay represents various attributes of a single employee. All relations (and thus tables) in a relational database have to adhere to some basic rules to qualify as relations. First the ordering of columns is immaterial in a delay. back up there can't be identical tuples or rows in a delay. And third each tuple ordain contain a hit determine for each of its attributes. A relational database contains multiple tables each similar to the one in the "flat" database copy. One of the strengths of the relational copy is that in principle any determine occurring in two different records (belonging to the same table or to different tables) implies a relationship among those two records. Yet in order to enforce explicit integrity constraints relationships between records in tables can also be defined explicitly by identifying or non-identifying parent-child relationships characterized by assigning cardinality (1:1. (0)1:M. M:M). Tables can also undergo a designated single evaluate or a set of attributes that can act as a "key" which can be used to uniquely identify each tuple in the table. A key that can be used to uniquely identify a row in a table is called a primary key. Keys are commonly used to join or combine data from two or more tables. For example an Employee table may contain a column named Location which contains a value that matches the key of a Location table. Keys are also critical in the creation of indices which aid fast retrieval of data from large tables. Any column can be a key or multiple columns can be grouped together into a compound key. It is not necessary to be all the keys in advance; a column can be used as a key even if it was not originally intended to be one. Relational operations :Users (or programs) communicate data from a relational database by sending it a ask that is written in a special language usually a dialect of SQL. Although SQL was originally intended for end-users it is much more common for SQL queries to.[ADVERTHERE]Related article:
http://worldofvarinder.blogspot.com/2007/12/what-is-database-computer-database-is.html
0 Comments:
No comments have been posted yet!
|
