Data Duplication
One major advantage of a
database system is the reduction in duplicated data. Before databases,
organisations often held many copies of the same data in different
departments, due to certain files being needed by more than one department.
For instance a record of a
customer's details may be held in the sales department, a record may also be
held of the same customer in the delivery department.
The main problem with this is
that it can lead to inconsistent data. As one department updates & maintains
data, but may not pass these updates onto the rest of the organisation. A
customer could move premises and the change of address may only be reflected
in the records held in one of the departments.
Obviously keeping duplicates of
data increases the storage space required & makes it difficult to keep the
data secure if it is sensitive information, as the information is held at
more than on location. Maintenance of this data also becomes near
impossible, as any changes to it must be done across the board.
Referential Integrity
Referential integrity is a
system of rules that ensures that relationships between records in related
tables are valid, and that you don't accidentally delete or change related
data.
It ensures that you cannot enter
a value in the foreign key field of a related that that doesn't exist in the
primary key of the table it is related to.
In a one to many relationship,
you cannot delete a record from the table on the one side of the
relationship if a matching record exists in the related table. You also
can't change the value of the key field in the table if it has matching
records in the related table.
Authentication
Database Authentication is to do
with using digital signatures so that the identity of the maintainer of the
database can be proved. Various methods of digital signing a database exist
such as using PGP public key authentication.
Transactions
A series of processing steps
that results in a specific function or activity being completed, ensuring
that a set of actions is treated as a single unit of work.
It is important to flush
transactions carried out on a database that is being held in memory onto the
file system. Databases are sometimes held in memory to allow work to be
carried out on them at a faster speed. Sometimes databases are not saved to
hard drive until the user exits from the system. This could mean lose of
data if the computer crashes before the user exits & saves.
Many databases can be set
however to automatically save the databases to the hard drive from memory,
without waiting for the user to hit save.
Recovery of Data
As with any computer system,
backups of data in a database are vital. Daily, weekly or even monthly
backups should be performed, depended on the importance of the data.
Database's can be relatively small, unless multimedia elements have been
stored in the database.
Tape Units, Floppy
Disk, Zip Disk & CD-RW (CD Rewriters) are all hardware devices on which
databases can be backed up onto. It really depends on the size & importance
of the database. CD-RW is probably the most reliable of the formats. It is
also the fastest and with the exception of tape units, can hold the most
data. A CD-RW can hold approx. 650Mb, while a Floppy Disk can hold only
1.44Mb and a Zip Disk can hold either 100Mb or 200Mb. Tape Units can hold an
incredible amount of data (20Gb to 80Gb), but are very slow.
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