What is Data Dictionary | Logical data independence and Physical data independence

DATA DICTIONARY:-

Data dictionary or Data repository or System catalog is an important part of the DBMS t contans data about data (metadata) i.e. it contains the real database descriptions used by the DBMS.

A data dictionary does not contain any actual data from the database; rather it contains information required for managing database and its various operations. Without a data dictionary, however, a database management system cannot access data from the database. In most DBMSs, the data dictionary is active and integrated. It means that the DBMS Checks the data dictionary every time the database is accessed. The data dictionary contains the following information:

Names of the tables.
Names and data types of attributes/data fields of each table.
Number of records in each table.
Constraints set on data fields.
Description of database users and their access rights.
Description about application programs.
Description about physical database design, such as storage structures, access paths etc.

A data dictionary is mainly used when finding information about users, objects, schemas and storage structures. Every time a data definition language (DDL) statement is issued, the data dictionary becomes modified.

Types of Data dictionaries: There are two types of data dictionaries:-

(i) Active Data dictionary

(ii) Passive Data dictionary

(i) Active Data dictionary :- A data dictionary that is automatically updated by the DBMS every time the database is accessed. It is also known as Integrated Data dictionary.

(ii) Passive Data Dictionary- Passive data dictionary is similar to active data dictionary however it is not automatically updated and usually requires a batch process to be run. lt s also known as Non-integrated Data dictionary. It is managed by the users of the database system and is modified whenever the structure of the database is changed.

Flashback:-

The activity of processing data using a computer is called Data Processing.
Data is raw material used as input and Information is processed data obtained as output of data processing.
Database can be defined as "A well organized collection of data that are related ina meaningful way, which can be accessed by different users but stored only once".
To maintain huge databases, W¢ need certain operations which helpin maintaining the in the database efficiently. The most commonly used operations performed on statbases the are: Insertion, Selection, Updation, Deletion and Sorting.
Manual database IS the recoru keeping systems in which human hole database without the being manage the support of computers. Whereas Computerized database is a database that can be managed or accessed by computers
File Svstem approach was an early attempt to computerize the manual filing system. It is hasically a collection of application programs that performed services for the end users. Each program within a ile based system defines and manages its own data. In this system, a large number of files are needed to perform various tasks .
In Database approach, instead ot disconnected files with redundant data, all data items are integrated with minimum amount of duplication and maintained only at one laration, In this approach. related data is shared by multiple application programs.
A database system is composed of four major components: Data, Hardware, Software and Users. These components coordinate with each other to form an effective database system.
DBMS (Database Management System) is a software system used to manage database and its various operations like insertion, deletion, updation and retrieval. enables users to store, modify and extract information from a database as per the requirements. It acts as an intermediator between the user and the database."
Data modeling is a way to structure and organize data so it can be used easily by databases.
Data integration means combining different types of data with different length in a | Single unit. In DBMS. we can store all the data with different data types and length in a Single repository (database).
DBMS, the database can have different aspects to reveal if seen from different levels 0T abstraction. The term abstraction means the amount of detail you want to hide. here are three levels of abstraction or views in the database: External level, Conceptual level and Internal level .
External level or user view is at the highest level of abstraction in DBMS architecture . This is targeted for the end users. In this view, only the restricted portion or atabase is available to end users as per his/her need .
Logical Data independence is the ability to change the logical or conceptual schema of the datahase without changing the extermal schema.
Physical Data Independence is the ability to change the physical or internal schema without changing the logical schema.
The DBA (Database Administrator) is a single person or a group of persons responsible for implementing the database system with in an organization.
Data dictionary contains data about data (metadata). It contains information required Sor managing database and its various operations.
Passive data dictionary is similar to active data dictionary however it is not sutomatically updated and usually requires a batch process to be run. It is managed by the users of the database system and is modified whenever the structure of the database is changed .

LOGICAL DATA INDEPENDENCE Vs. PHYSICAL DATA INDEPENDENCE :-

Logical Data Independence:-

It is concerned with the structure of the data.
It is very difficult as the retrieving of data is very much dependent on the logical structure of data.
Application program need not be changed if new fields are added or deleted from the databse .
It is concerned with the conceptual schema .

PHYSICAL DATA INDEPENDENCE:-

It is concerned with storage of the data .
It is easy to retrieve .
Physical database is concerned with the change of the storage device .
It is concerned with the internal schema .

What is Schema in DBMS ?

Schema represents the overall structure or design of the database. It is a framework into hich the values of the data items data fields are fitted. It is important to distinguish aween the description of the database and the database itself. The description of the database is the databasc schema. The schema Is specitied during the database design process dis not expected to change frequently. However the actual data in the database may change foaently. For example: It changes every ime whenever we insert a new record in the database.

In other words, schema means the overall plan or scheme that describes the records and relationships among them existing at a particular level in the DBMS architecture.

TYPES OF SCHEMA :-

There are three different types of schema in the database corresponding to each data view of database:
i) External schema
ii) Conceptual schema
iii) Internal schema
i) External schema: The external view is described by means of a schema called e DBMS external schema that correspond to different views of the data. It specifies different views that re: enable different users of the data to see it in different ways.

ii) Conceptual schema: The conceptual view is defined by conceptual schema, which describes all the entities, attributes and relationship together with integrity constraints.

iii) Internal schema: Internal view is defined by internal schema. It describes how the data will be physically stored and accessed, using the facilities provided by a particular DBMS .

What is SubSchema in DBMS ?

  Subschema is the subset of schema. It inherits the same property as that of a schema. The subschema refers to user's view of fields that he or she needs from the database. As there exist multiple users, each accessing some portion of the database so there can exist different sub-schemas for a given schema.
Sub-schemas are mainly used due to the following reasons:-
Sub-schemas provide different views of the data to the user and the programmer, who do not need to know all the data contained in the entire database.

Sub-schemas enhance security factors and prohibit data compromise.

Sub-schemas give support to the DBA while assuring data integrity .
WHAT IS DATA INDEPENDENCE:-

Database management system (DBMS) supports the concept or data independence sine functions and res it represents a system for managing data separately from the programs that use the data, The three-level architecture of DBMS provides the concept of data independence, whic means that upper-levels are unaffected by changes to lower-levels. In other words, it allow changes to the structure of a database without requiring application programs or users DBA is organiza DBA databas= make any change in the way they access the data.
Types of Data Independence:-
   (a) Logical Data Independence: The ability to change the logical or conceptual schem of database without changing the external schema is called logical data independence. Fo example, the addition or removal of an attribute or field to the conceptual schema should b possible without having to change existing external schema or having to re'write existin application programs. data DBA lost th�DBA datab
    (b) Physical Data Independence: The ability to change the physical or internal without schem changing the logical schema is called physical data independence. For example,i change to the internal schema, such as using different storage devices should be without possibk having to change the conceptual or external schemas .