What is a database management system?
A database management system (DBMS) is a software package which is used to create, manipulate and present data from electronic databases.
Examples of DBMSs include Microsoft Access and Filmmaker Pro.
· Storage of paper records was very bulky
· Easy to mis-file a record, or records to be lost or damaged
· Data often duplicated in several records
· Keeping records up-to-date was difficult and time consuming, and often resulted in data inconsistency, where duplicated values were updated in one record but not in others
· Many people employed to maintain the records, which was costly
· Searching for records was time consuming
· Producing reports, such as sorted lists or data collated from several sources, was extremely time consuming, if not impossible
Benefits of computerized databases
· Searching, sorting and calculating operations can be performed much more quickly and easily.
· Information is more easily available to users, due to improved methods of data retrieval.
· Data integrity is improved resulting in more accurate information.
Types of computerized database
· Flat file – Tabular (All data in a single table)
Flat file databases
Limitations of flat file databases
· Data is very likely to be duplicated.
· The duplication of data leads to the possibility of data inconsistency.
· It is not possible to store information about a member without entering details of a DVD. This is called an insertion anomaly.
· Removing a DVD from the database may remove the only record which stores details of a Member. This is called a deletion anomaly.
· A relational database stores data in more than one table.
· The idea is to ensure that data is only entered and stored once, so removing the possibility of data duplication and inconsistency.
Entities, Attributes and Instances
· An entity represents a person or object e.g. Member, DVD Rental
· Each entity has a set of attributes which describe examples or instances of that entity.
o The attributes of the DVD Rental entity are code, title, cost, date out, date due and member number
o The attributes of the Member entity are member number, name and telephone number.
Three types of relationship:
A key is a field, or set of fields, whose values uniquely identify a record.
In any table, there may be more than one field or set of fields, which can uniquely identify each record—these are called candidate keys.
The candidate key which is chosen to be used is called the primary key.
· Member Number is a candidate key for the Member entity
· MEMBER (Member Number, Name, Telephone Number)
· DVD Code is a candidate key for the DVD Rental entity
· DVD RENTAL (DVD Code, Title, Cost, Date Out, Date Due, *Member Number)
· Member Number is called a foreign key.
· A foreign key is a field which is not a primary key in its own table, but is a primary key in another table.
· Member Number is a foreign key in the DVD table, because it is the primary key in the Member table.
· Here is the data model:
o MEMBER(Member Number, Name, Telephone Number)
o DVD RENTAL(DVD Code, Title, Cost, Date Out, Date Due, *Member Number)