Lesson 2.3: Levels of Database Architecture Flashcards
ANSI Standards Planning and Requirements Committee (SPARC) architecture is divided into three levels
External Level
Conceptual Level
Internal Level
user’s view of the database
External Level
the community view of the database
Conceptual Level
physical representation of the database on the computer
Internal Level
part of the database that is relevant to the user and is concerned with the way the data is seen by individual users
External Level
describes what data is stored in the database and the relationships among the data
Conceptual Level
describes how the data is stored in the database and is concerned with the way the data is physically stored
Internal Level
conceptual level represents
- all entities, their attributes and their relationships
- constraints on the data
- semantic information about the data
- security and integrity information
internal level represents
- storage space allocation for data and indexes
- record descriptions for storage (with stored sizes for data items)
- record placement
- data compression and data encryption techniques
reasons of separation
- Each user should be able to access the same data, but may have different view of the data.
- User should not have to deal directly with physical data storage detail.
- The DBA should be able to change the database storage structure w/o affecting the users’ view
- The internal structure of the database should be unaffected by changes to the physical aspects of storage.
an individual user level
external level
in external level, all such languages will include a ___
data sublanguage (DSL)
subset of the total language that is concerned specifically with database objects nad operations, embedded within the corresponding host language and is responsible for providing various nondatabase facilities
data sublanguage (DSL)
what particular DSL is supported?
SQL
A DSL is a combination of at least two subordinate languages
- Data Definition Language (DDL)
- supports the definition or declaration of database objects
- Data Manipulation Language (DML)
- supports the manipulation or processing of such objects
content of the database as seen by some particular user that consists of many occurrences of each of many types of external record
external view
written using the DDL portion of the user’s DSL.
external schema
a representation of the entire information content of the database in a form that is somewhat abstract in comparison with the way in which the data is physically stored and is a view of the total database content that consists of many occurrences of each of many types of conceptual record
Conceptual view / conceptual level?
The conceptual view is defined by means of the ___
conceptual schema
includes definitions of each of the various conceptual record types
conceptual schema
written using another DDL, the conceptual DDL
conceptual schema
a schema intended to include the a great many additional features, such as security and integrity constraints
conceptual schema
t/f A conceptual record is not necessarily the same as either an external record or a stored record.
true
low-level representation of the entire database
internal level
consists of many occurrences of each of may types of internal record
internal level
internal view is still at one remove from the physical level since it does not deal in terms of physical records, also called _____ or pages - nor with any device specific considerations such as cylinder or track sizes.
blocks
described by means of the internal schema, which not only defines the various stored record types but also specifies what indexes exist, how stored fields are represented, what physical sequence the stored records are in and so on.
internal view
the database system architecture involves certain mappings
The conceptual/internal mapping
The conceptual/external mapping
defines the correspondence between the conceptual view and the stored database
It specifies how conceptual records and fields are represented at the internal level
If the structure of the stored database is changed - that is if a change is made to the storage structure definition - then the conceptual/internal mapping must be changed accordingly, so that the conceptual schema can remain invariant.
The conceptual/internal mapping
- defines the correspondence between a particular external view and the conceptual view
- In general, the differences that can exist between these two levels are analogous to those that can exist between the conceptual view and the stored database.
The conceptual/external mapping
A system provides ____________- if users and user programs are immune to changes in the physical structure of the stored database.
physical data independence
Analogously, a system provides ________________ if users and user programs are also immune to changes in the logical structure of the database.
logical data independence
