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create a third table which will be intermediate between the 
two tables with many to many relationships. for instance if 
the two table were "student" and "room", create a third 
table called "student_room". in this table put PIDs of the 
two tables. uh?

A new table is created with two foreign keys. This 
arrangement is necessary to make the database efficient.

Assosication shows the relationship between classes

Association is semantic relationship used between objects 
by links.

Activity Diagram mainly represent process flows captured in
system.

Sequence diagram mainly represent interaction between
different objects.It is time ordered means exact
interactions between objects is represented step by step.

Sequence diagram describes the behavior of several objects 
in a single use case.
activity diagrams are good at showing the general sequence 
of actions for several objects and use cases.

In UML Activity diagram is used for functional modeling
while sequence diagram is used to dynamic modeling purpose.

A typical workflow can be repersented by activity diagram 
while sequence diagram is more about exploring into use 
case by showing intercation between objects.

Note: Activity diagram also repersents an object role in 
workflow like sequence diagram but level of repersenation 
is not dynamic like sequence diagram repersentation.

for example: Object lifeline and sequence of interaction
is detailed in sequence diagram while activity diagram does 
not show such detail about an object.

include:
     This relationship occurs when you have a chunk of 
behaviour that is similar across more than one use case.
extends:
     This is similar to generalization but with more rules 
to it.

Uses : When you are using the functionality for any actor 
i.e. nothing but the use case

extend: When use case is alloted to the actor as per client 
requirement then programmer will extend the functionality 
as per the program needs.

includes : It is same as extends, here to fulfill the 
requirement of client, uses cases of actor has been 
designed but for program working it is necessary to extend 
or includes the functionality
For ex:
Suppose teacher is assigning and viewing grades of 
students. As programmer point of view for viewing grade it 
is neccessary to save (extending functionality) to view.

First of all I like to mention that "extends" & "includes" 
are use case relationships, and "uses" relationship is now 
excluded in UML 2.0 standard. 

Differences b/w includes & extends:
1> The included use case is always used to augment the 
executing use case while the extension use case might be 
used to augment the executing use case.
2> The executing use case decides when to call the included 
use case. The included use case is unaware of the base use 
case while The extension use case decides when it will 
insert itself into the execution of the base use case. The 
base use case is unaware of the extension.
3> The relationship arrow is drawn from the executing use 
case to the included use case. The base of the arrow 
indicates that the base use case directs the included use 
case to execute while the relationship arrow is drawn from 
the extension use case to the executing use case. The base 
of the arrow indicates that the extension use case is 
making the decision whether to interrupt the executing use 
case.

Sequence diagrams emphasize the time ordering of messages, 
whereas collaboration diagrams depict more of an 
organizational structure and are more space efficient. Many 
UML tools will automatically convert from one diagram type 
to the other.The difference is more in how the information 
is presented than in the underlying semantics of the 
diagram.

Composition : Defines a strong-coupled relationship between
two entities, where the one entity is part of another, and
both need each other for their existence. e.g. Human body
and the Heart.

Aggregation : Defines a weak-coupled relationship between
two entities, where one entity could be part of another, but
either can exist without the other, independantly. e.g.
School and teacher.

"Aggregation differs from ordinary composition in that it 
does not imply ownership. In composition, when the owning 
object is destroyed, so are the contained objects. In 
aggregation, this is not necessarily true. For example, a 
university owns various departments (e.g., chemistry), and 
each department has a number of professors. If the 
university closes, the departments will no longer exist, 
but the professors in those departments will continue to 
exist. Therefore, a University can be seen as a composition 
of departments, whereas departments have an aggregation of 
professors. In addition, a Professor could work in more 
than one department, but a department could not be part of 
more than one university."

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