Databases
Key
Definitions
§ A file is a collection of sets of similar
data called records
§ A table is another name for a file
§ A tuple is another name for a record
§ An item is an item of data within a
recorded
§ A field is the area where items are
stored
§ An attribute is another name for a field
§ A database is a series of related files,
called tables
§ The primary or key field is a unique field in a record
§ A foreign key is the field which contains
a link to another field
§ A secondary key is a field in a table
that can be used to access the data in different ways
Flat and Relational databases
§ A flat file database is a single
dimensioned collection of data
§ There are
several limitations to flat file databases:
o
Separation and isolation of data
o
Duplication of data
o
Data dependence
o
Incompatibility of files
o
Fixed queries and the proliferation of programs
§ A Relational database links different
fields between tables, which increases the efficiency and manages resources
better through a more dynamic approach.
Arranging
data
§ A normal form is the name given to how
data in a database is arranged
§ First normal form (1NF) is where each table has no repeating
groups
§ Second normal form (2NF) is where the values of the attributes
are all dependant on the primary key
§ Third normal form (3NF) is where no attributes are predictable
because of one of the other attributes
Analysing and implementing database design
§ The primary
key indicating a unique tuple by which a record can be identified, must be
shown by putting it in brackets and underlining it.
§ A many-to-one or one-to-many relationship is where there is one record linking to
many other records in another table.
§ A many-to-many relationship is where many
records link to many more records.
§ A link table is the table which is used
to store many-to-many relationships
§ A link entity is another name for a link
table
Database management systems
§ A database management system (DBMS) is a
piece of software that allows a database to be managed and controlled.
§ Data description language (DDL) is included in the DBMS. It is used to
define tables and the data types and data structures for files, and any
restrictions.
§ The schema is the design that is created by
the DDL
§ Different
users with different privileges will be able to see different parts of the
database, called the subschema.
§ Data manipulation language (DML) is also included in the DBMS. It will
allow users to manipulate the data. Depending on their rights, different users
will be able to do different things with the data.
§ A data dictionary contains all file
descriptions on the structure and type of data, it is
stored within the DBMS.
§ With the use
of various tools, the DBMS allows data to be presented in a variety of
different ways:
o
Internal
level is
the view of the entire database as it is stored in the system
o
Conceptual
level gives
a single usable view of all the data in the database
o
External
level is
where data is arranged according to the user requirements and rights
§ SQL is a simple language that allows users to set up their own queries
on a database