Notes > Foundations of Computing > Database Systems
A database is a multi-dimensional collection of data which contains links between the entities that exist within it so that information can be accessed from various perspectives. Databases can be used to store information about people (e.g. students at a school) or objects (e.g. books in a library).
An entity can be defined as anything about which information is stored. For example, possible entities could be a person, physical object, or even an event. An attribute is a descriptive property or characteristic of an entity. Entities can be arranged into entity classes (or entity types) where all of the entities within that type share certain attributes.
Implementing database technology can make it easier to access information in a system. If a standard file-processing system is used, data is accessed directly from file, whereas a database system involves the accessing of data through a Database Management System (DBMS).
A company may have several file processing systems which are used to access certain files. For example, there may be a customer processing application which is used to enter details regarding customers of a company. This data is then stored to a single customer file. There may be many other files with associated processing applications existing within the whole company's system.
By using a DBMS, all the different processing applications can be used to interact with the DBMS, which then interacts with a single database which holds all the data relating to the entities defined within the system.
Expert Systems involve the idea of "intelligent" databases. These can assist humans with decision making within a specific area or field. These Expert Systems are built by gaining knowledge from an expert then expressing this knowledge as set of logical rules. This set of logical rules is known as the knowledge base.
A database "schema" is how all the data tables and indexes are related. Databases remove the problem of data duplication that can easilt occur across flat files. A relational database is defined as being a database where data is held in table with uniquely named columns. Each row contains a unique set of data. The order of the rows and columns is irrelevant in relational databases.
A "key" is a value that is unique for every record. These keys can therefore be used to identify any record, given a specified key. A primary key is the main record identifier.
Relationships within a relational database are created by storing primary key values in another table. This table is then known as the "relation". A primary key that has been stored in another table is known as a "foreign key" when in this other table.
SQL (Structure Query Language) is used to access relational databases. SQL extracts a subset of the database and creates a "view". This view is a collection of data from different tables relevant to the specified application.
A DBMS (Database Management System) manages corporate data and allows the database to be accessed by applications. It is important to distinguish between a DBMS and the File Store Manager (e.g. DOS) as the File Store Manager is a component of the Operating System and manages the actual file allocation on disc and accessing of that data. The DBMS accesses the hard disc storage through the File Storage Manager.
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