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Wednesday, 25 February 2015
Monday, 23 February 2015
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Wednesday, 12 November 2014
post fix expression
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data structure 2nd sessional course
Sessional course data structure:
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Double link list( insertion , deletion , display)
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Circular link list ( insertion , deletion , display)
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Stack ( insertion , deletion , display)
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Queue ( insertion , deletion , display)
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Expression evaluation/polish notation(
infix,prefix,postfix)e.g table from diagram
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Tree(theory)
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Tree traversial.
Sunday, 9 November 2014
Cardinality of Relationships
Cardinality is the number of entity instances to which another entity
set can map under the relationship. This does not reflect a requirement
that an entity has to participate in a relationship. Participation is
another concept.
One-to-one: X-Y is 1:1 when each entity in X is associated with at most one entity in Y, and each entity in Y is associated with at most one entity in X.
One-to-many: X-Y is 1:M when each entity in X can be associated with many entities in Y, but each entity in Y is associated with at most one entity in X.
Many-to-many: X:Y is M:M if each entity in X can be associated with many entities in Y, and each entity in Y is associated with many entities in X (“many” =>one or more and sometimes zero)
One-to-one: X-Y is 1:1 when each entity in X is associated with at most one entity in Y, and each entity in Y is associated with at most one entity in X.
One-to-many: X-Y is 1:M when each entity in X can be associated with many entities in Y, but each entity in Y is associated with at most one entity in X.
Many-to-many: X:Y is M:M if each entity in X can be associated with many entities in Y, and each entity in Y is associated with many entities in X (“many” =>one or more and sometimes zero)
Keys in DBMS
Super key: An attribute or set of attributes that uniquely identifies an entity–there can be many of these
Composite key:A key requiring more than one attribute
Candidate key: a superkey such that no proper subset of its attributes is also a superkey (minimal superkey – has no unnecessary attributes)
Primary key: The candidate key chosen to be used for identifying entities and accessing records. Unless otherwise noted “key” means “primary key”
Alternate key: A candidate key not used for primary key
Secondary key: Attribute or set of attributes commonly used for accessing records, but not necessarily unique
Foreign key: An attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.
Composite key:A key requiring more than one attribute
Candidate key: a superkey such that no proper subset of its attributes is also a superkey (minimal superkey – has no unnecessary attributes)
Primary key: The candidate key chosen to be used for identifying entities and accessing records. Unless otherwise noted “key” means “primary key”
Alternate key: A candidate key not used for primary key
Secondary key: Attribute or set of attributes commonly used for accessing records, but not necessarily unique
Foreign key: An attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.
Entities and Attributes
Entity Type:It is a set of
similar objects or a category of entities that are well defined
- A rectangle represents an entity set
- Ex: students, courses
- We often just say “entity” and mean “entity type”
Attribute:It describes one
aspect of an entity type; usually [and best when] single valued and indivisible
(atomic)
- Represented by oval on E-R diagram
- Ex: name, maximum enrollment
Types of Attribute:
Simple and Composite
Attribute
Simple attribute that consist of a single atomic value.A simple
attribute cannot be subdivided. For example the attributes age, sex etc are
simple attributes.
A composite attribute is an attribute that can be further
subdivided. For example the attribute ADDRESS can be subdivided into street,
city, state, and zip code.
Simple Attribute: Attribute that consist of a single atomic value.
Example: Salary, age etc
Composite Attribute
: Attribute value not atomic.
Example : Address : ‘House_no:City:State
Name : ‘First Name: Middle Name: Last Name’
Example : Address : ‘House_no:City:State
Name : ‘First Name: Middle Name: Last Name’
Single Valued and Multi
Valued attribute
A single valued attribute can have only a single value. For example
a person can have only one ‘date of birth’, ‘age’ etc. That is a single valued
attributes can have only single value. But it can be simple or composite
attribute.That is ‘date of birth’ is a composite attribute , ‘age’ is a simple
attribute. But both are single valued attributes.
Multivalued attributes can have multiple values. For instance a
person may have multiple phone numbers,multiple degrees etc.Multivalued
attributes are shown by a double line connecting to the entity in the ER diagram.
Single Valued Attribute: Attribute that hold a single value
Example1: Age
Exampe2: City
Example3:Customer id
Example1: Age
Exampe2: City
Example3:Customer id
Multi Valued Attribute: Attribute that hold multiple values.
Example1: A customer can have multiple phone numbers, email id’s etc
Example2: A person may have several college degrees
Example1: A customer can have multiple phone numbers, email id’s etc
Example2: A person may have several college degrees
Stored and Derived
Attributes
The value for the derived attribute is derived from the stored
attribute. For example ‘Date of birth’ of a person is a stored attribute. The
value for the attribute ‘AGE’ can be derived by subtracting the ‘Date of
Birth’(DOB) from the current date. Stored attribute supplies a value to the
related attribute.
Stored Attribute: An attribute that supplies a value to the related
attribute.
Example: Date of Birth
Example: Date of Birth
Derived Attribute: An attribute that’s value is derived from a
stored attribute.
Example : age, and it’s value is derived from the stored attribute Date of Birth.
Example : age, and it’s value is derived from the stored attribute Date of Birth.
