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IVE Database
Chapter 2 - Database Concepts and Architecture
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Chapter 2 -
Architecture for a Database System:
1. Topics
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Database System Architecture
Program-Data Independence
Data and Metadata
Data Dictionary
2. Database System Architecture
The ANSI/SPARC architecture is divided into three levels, known as the internal,
conceptual, and external level, respectively. Broadly speaking,
(An Introduction to Database Systems, C.J. Date, 7th edition, p. 33)
a) The internal level (also known as the physical level):
The one closest to physical storage-i.e., it is the one concerned with the way
the data is physically stored;
Specifications for how data from a conceptual schema are stored in secondary
storage.
b) The external level (also known as the user logical level):
It is the one closest to the users- i.e. it is the one concerned with the way the
data is seen by individual users. – User View
c) The conceptual level (also known as the community logical level, or
sometimes just the logical level, unqualified):
A level of indirection between the other two. A detailed, technology
independent specification of the overall structure of a database – the Data
Model.
The three levels of the architecture
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An example of the three levels
3. Program-Data Independence
The separation of data description (metadata) from the application programs that
use the data is called data independence. With the database approach, data
descriptions are stored in a central location called the repository. This property
of database systems allows an organization's data to change and evolve (within
limits) without changing the application programs that process the data.
4. Data and Metadata
a) Data
Refer to meaningful facts, text, graphics, images, sound and video segments.
b) Metadata
Metadata are data that describe the properties or characteristics of other data.
Some of these properties include data definitions and data structures.
Some sample metadata for the Class Roster (Figure 1-1a) are listed in Table
1-1 (McFadden, Modern Database Management, fifth edition, P. 7). For each
data item that appears in the Class Roster, the metadata show the data item
name, the data type, length, minimum and maximum allowable values (where
appropriate), and a brief description of each item. Metadata describe the
properties of data but do not include that data.
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Chapter 2 - Database Concepts and Architecture
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Fig 1-1a
Table 1-1 Example Metadata for Class Roster
Data Item
----------------------------------Name
Type
Course
Alphanumeric
Section
Integer
Semester
Alphanumeric
Name
Alphanumeric
ID
Integer
Major
Alphanumeric
GPA
Decimal
Length
30
1
10
30
9
4
3
Value
----------------------Min
Max
Description
Course ID and name
1
9
Section number
Semester and year
Student name
Student ID (SSN)
Student major
0.0
4.0
Student grade point average
5. Data Dictionary
A repository of information about a database which documents data elements of a
database.
An integral part of relational DBMSs is the data dictionary, which stores metadata,
or information about the database, including attribute names and definitions for
each table in the database.
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Data dictionary -what should be in it?
Data dictionary may cover the whole organisation, a part of the organisation or a database. In
its simplest form, the data dictionary is only a collection of data element definitions,
according to descriptions below. More advanced data dictionary contains database schema
with reference keys, still more advanced data dictionary contains entity-relationship model of
the data elements or objects. The term "data element" is used below. It is the same concept as
"data object" or "object" in some database texts.
Data element definitions
.Data element number
Data element number is used in the technical documents.
.Data element name (caption)
Commonly agreed, unique data element name from the application domain. This is the real
life name of this data element.
.Short description
Description of the element in the application domain.
.Security classification of the data element
Organisation-specific security classification level or possible restrictions on use. This may
contain technicallin~ to security systems.
.Related data elements
List of closely related data element names when the relation is important.
.Field name(s)
Field names are the names used for this element in computer programs and database schemas.
These are the technical names, often limited by the programming languages and systems.
.Code format
Data type (characters, numeric, etc.), size and, if needed, special representation. Common
programming language notation, input masks, etc. can be used.
.Null value allowed
Null or non-existing data value may be or may not be allowed for an element. Element with
possible null values needs special considerations in reports and may cause problems, if used
as a key.
.Default value
Data element may have a default value. Default value may be a variable, like current date and
time of the day (DoD).
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.Element coding (allowed values) and intra-element validation details or reference to
other documents
Explanation of coding (code tables, etc.) and validation rules when validating this element
alone in the application domain.
.Inter-element validation details or reference to other documents
Validation rules between this element and other elements in the data dictionary.
.Database table references
Reference to tables the element is used and the role of the element in each table. Special
indication when the data element is the key for the table or a part of the key.
.Definitions and references needed to understand the meaning of the element
Short application domain definitions and references to other documents needed to understand
the meaning and use of the data element.
.Source of the data in the element
Short description in application domain terms, where the data is coming. Rules used in
calculations producing the element values are usually written here.
.Validity dates for the data element definition
Validity dates, start and possible end dates, when the element is or was used. There may be
several time periods the element has been used.
.History references
Date when the element was defined in present form, references to superseded elements, etc.
.External references
References to books, other documents, laws, etc.
.Version of the data element document
Version number or other indicator. This may include formal version control or configuration
management references, but such references may be hidden, depending on the system used.
.Date of the data element document
Writing date of this version of the data element document.
.Quality control references
Organisation-specific quality control endorsements, dates, etc.
.Data element notes
Short notes not included in above parts.
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