External Level
External The users’ view of the database. This level describes that part of the
level database that is relevant to each user.
The external level consists of a number of different external views of the database. Each
user has a view of the ‘real world’ represented in a form that is familiar for that user. The
external view includes only those entities, attributes, and relationships in the ‘real world’
that the user is interested in. Other entities, attributes, or relationships that are not of interest
may be represented in the database, but the user will be unaware of them.
In addition, different views may have different representations of the same data. For
example, one user may view dates in the form (day, month, year), while another may view
dates as (year, month, day). Some views might include derived or calculated data: data
not actually stored in the database as such, but created when needed. For example, in the
DreamHome case study, we may wish to view the age of a member of staff. However, it
is unlikely that ages would be stored, as this data would have to be updated daily. Instead,
the member of staff’s date of birth would be stored and age would be calculated by the
DBMS when it is referenced. Views may even include data combined or derived from
several entities. We discuss views in more detail in Sections 3.4 and 6.4.
Conceptual Level
Conceptual The community view of the database. This level describes what data
level is stored in the database and the relationships among the data.
The middle level in the three-level architecture is the conceptual level. This level contains
the logical structure of the entire database as seen by the DBA. It is a complete view of the
data requirements of the organization that is independent of any storage considerations.
The conceptual level represents:
n all entities, their attributes, and their relationships;
n the constraints on the data;
n semantic information about the data;
n security and integrity information.
The conceptual level supports each external view, in that any data available to a user must
be contained in, or derivable from, the conceptual level. However, this level must not contain
any storage-dependent details. For instance, the description of an entity should contain
only data types of attributes (for example, integer, real, character) and their length
(such as the maximum number of digits or characters), but not any storage considerations,
such as the number of bytes occupied.
Internal Level
Internal The physical representation of the database on the computer. This level
level describes how the data is stored in the database.
The internal level covers the physical implementation of the database to achieve optimal
runtime performance and storage space utilization. It covers the data structures and file
organizations used to store data on storage devices. It interfaces with the operating system
access methods (file management techniques for storing and retrieving data records) to
place the data on the storage devices, build the indexes, retrieve the data, and so on. The
internal level is concerned with such things as:
n storage space allocation for data and indexes;
n record descriptions for storage (with stored sizes for data items);
n record placement;
n data compression and data encryption techniques.
Below the internal level there is a physical level that may be managed by the operating
system under the direction of the DBMS. However, the functions of the DBMS and the
operating system at the physical level are not clear-cut and vary from system to system.
Some DBMSs take advantage of many of the operating system access methods, while
others use only the most basic ones and create their own file organizations. The physical
level below the DBMS consists of items only the operating system knows, such as exactly
how the sequencing is implemented and whether the fields of internal records are stored as
contiguous bytes on the disk.
External The users’ view of the database. This level describes that part of the
level database that is relevant to each user.
The external level consists of a number of different external views of the database. Each
user has a view of the ‘real world’ represented in a form that is familiar for that user. The
external view includes only those entities, attributes, and relationships in the ‘real world’
that the user is interested in. Other entities, attributes, or relationships that are not of interest
may be represented in the database, but the user will be unaware of them.
In addition, different views may have different representations of the same data. For
example, one user may view dates in the form (day, month, year), while another may view
dates as (year, month, day). Some views might include derived or calculated data: data
not actually stored in the database as such, but created when needed. For example, in the
DreamHome case study, we may wish to view the age of a member of staff. However, it
is unlikely that ages would be stored, as this data would have to be updated daily. Instead,
the member of staff’s date of birth would be stored and age would be calculated by the
DBMS when it is referenced. Views may even include data combined or derived from
several entities. We discuss views in more detail in Sections 3.4 and 6.4.
Conceptual Level
Conceptual The community view of the database. This level describes what data
level is stored in the database and the relationships among the data.
The middle level in the three-level architecture is the conceptual level. This level contains
the logical structure of the entire database as seen by the DBA. It is a complete view of the
data requirements of the organization that is independent of any storage considerations.
The conceptual level represents:
n all entities, their attributes, and their relationships;
n the constraints on the data;
n semantic information about the data;
n security and integrity information.
The conceptual level supports each external view, in that any data available to a user must
be contained in, or derivable from, the conceptual level. However, this level must not contain
any storage-dependent details. For instance, the description of an entity should contain
only data types of attributes (for example, integer, real, character) and their length
(such as the maximum number of digits or characters), but not any storage considerations,
such as the number of bytes occupied.
Internal Level
Internal The physical representation of the database on the computer. This level
level describes how the data is stored in the database.
The internal level covers the physical implementation of the database to achieve optimal
runtime performance and storage space utilization. It covers the data structures and file
organizations used to store data on storage devices. It interfaces with the operating system
access methods (file management techniques for storing and retrieving data records) to
place the data on the storage devices, build the indexes, retrieve the data, and so on. The
internal level is concerned with such things as:
n storage space allocation for data and indexes;
n record descriptions for storage (with stored sizes for data items);
n record placement;
n data compression and data encryption techniques.
Below the internal level there is a physical level that may be managed by the operating
system under the direction of the DBMS. However, the functions of the DBMS and the
operating system at the physical level are not clear-cut and vary from system to system.
Some DBMSs take advantage of many of the operating system access methods, while
others use only the most basic ones and create their own file organizations. The physical
level below the DBMS consists of items only the operating system knows, such as exactly
how the sequencing is implemented and whether the fields of internal records are stored as
contiguous bytes on the disk.
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