Schema:
Design of a database is called the schema. A
database schema is the skeleton structure that represents the logical view of
the entire database. It defines how the data is organized and how the relations
among them are associated. It is the collection of named objects. The schema
contains names of tables, columns of each table, datatype, triggers, functions,
views, stored procedures and other objects are included in the schema. A
database schema can be represented by using the visual diagram. That diagram
shows the database objects and relationship with each other. A database schema
is designed by the database designers to help programmers whose software will
interact with the database. The process of database creation is called data
modeling. The changes in a schema are not applied so frequently, but
occasionally changes need to be applied as the requirements of application
changes.
For Example: In the following diagram, we have a
schema that shows the relationship between three tables: Course, Student and
Section. The diagram only shows the design of the database, it doesn’t show the
data present in those tables. Schema is only a structural view (design) of a
database as shown in the diagram below.
Schema
is of three types: Physical schema, logical schema and view schema.
The
design of a database at physical level is called physical schema. It describes
how the data stored on the disk or the physical storage.
Design
of database at logical level is called logical schema,
programmers and database administrator work at this level. At this level data
can be described as certain types of data records gets stored in data
structures.
Design
of database at view level is called view schema.
This generally describes end user interaction with database systems.
Subschema:
A
subschema provides a view of the database as seen by an application program.
This view is often a subset of the complete schema definition. A subschema is
used at run time to provide the DBMS with a description of those portions of the
database that are accessible to the application program. The subschema allows
the user to view only that part of the database that is of interest to him. The
subschema defines the portion of the database as seen by the application
programs and the application programs can have different view of data stored in
the database.
The
subschema identifies the areas, records, elements, and sets which are
accessible. The subschema identifies the Data Manipulation Language (DML)
functions which can be performed. The different application programs can change
their respective subschema without affecting other's subschema or view.
Instance:
The
data stored in database at a particular moment of time is called instance of
database. It is a snapshot where the current state or occurrence of a
database is framed at that moment. Database schema defines the variable
declarations in tables that belong to a particular database; the value of these
variables at a moment of time is called the instance of that database. Instance
changes very frequently, whenever data is removed or added in the database.
For example: We have a table student in the
database, today the table has 10 records, so today the instance of the database
has 10 records. We are going to add another 10 records in this table by
tomorrow so the instance of database tomorrow will have 20 records in table. In
short, at a particular moment the data stored in database is called the
instance, that changes over time when we add or delete data from the database.
Difference between Schema and Instance:
- A schema is the design representation of a database whereas instance is the snapshot of a database at a particular moment.
- Instance changes very frequently, whenever data is removed or added in the database. As against, the changes in schema occurs rarely.
-profshardulp.patil@gmail.com
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