Entity Relationship Diagrams Video Lecture | SSC CGL Tier 2 - Study Material, Online Tests, Previous Year

all right moving right along the next
type of diagram we'll cover is called
the entity relationship diagram or ER
diagram for short this kind of diagram
depicts entities which are essentially
things and the relationships between
them along with attributes of those
entities and relationships we use them
extensively in business analysis
primarily as a way to start thinking of
modeling data you can't really
understand what they are before you see
it so let's take a look at the building
blocks of an ER diagram and then
assemble one together well actually make
a very small model for udemy which I
think we're all familiar with enough
entities are the first component of any
ER diagram and they're represented by
rectangles like this as with just about
any shape you see in diagramming we have
entered some text into it to show what
the entity represents by convention the
name of the entity is a noun and it's
also singular in number this doesn't
mean that you can only have one indeed
unity has loads of courses it's just the
convention that we that we use er
diagrams also have relationships and
they're represented by diamond shapes
there's an example just as you would
expect we also place text in the diamond
shape that represents the type of
relationship now here is a rule of ER
diagrams a relationship is always
between two entities two rectangles so
let's put together our first picture
that actually makes any sense ah great
okay great
here we have a user entity studying a
course entity and that's coincidentally
what you're doing right now however
there is one missing piece in this
diagram in order for it to be well
formed and that is what we call
cardinality let's display that on the
screen and then
talk about it okay so you'll notice that
there is now an M next to the user
entity and next to the course entity
okay what do these mean in this case it
means in short that any number of users
can study any number of courses the M
and n are simply variables that
represent any whole number from 0 to
infinity there are a couple other
variants that we could put in here as
well we could make the N a 1 and that
would mean that a course can be studied
by any number of users but a single user
can only study one course alternatively
we can make the M a 1 and that would
represent there is only one user but she
can study any number of courses
generally we have three different types
of relationships we have one-to-one
relationships one-to-many relationships
and many to many relationships
one-to-one relationships are when there
is a single instance of one entity which
has a relationship to a single instance
of another entity one-to-many
relationships are when there is a single
instance of identity which can have a
relationship with many instances of
another entity and many-to-many
relationships are when there are many
instances of an entity that are related
to many instances of another entity
that's a mouthful alright since there
are many users of unity and unity has
many courses available and since users
can study as many courses as they like
this relationship is considered many to
many let's illustrate this by adding
another relationship between these two
entities like this okay here we have a
teacher's relationship that's new
because instructors are also users here
I've made this a one-to-many
relationship using the assumption that
only one instructor can teach any
particular course I'm not sure if that's
true or not but let's just make that
assumption for purposes of this model I
realized that getting cardinality right
can be challenging so I'll give you a
heuristic for it rule of thumb when you
have two entities like user of course
that are joined by a studies
relationship ask yourself these
questions
get a user study multiple courses or
only one if it's multiple then we want
to put an end next to the course if it's
only one put a one next to the course
second can of course be studied by
multiple users or only one if it's
multiple then put an M next to user if
only one put a 1 next to user and that's
how we ended up with the cardinality
rules that are in this diagram there is
one more attribute that we commonly
place in ER diagram so those are while
they're called attributes they are
essentially descriptions of you can
think of them as adjectives you there
they're their attributes of an entity or
of a relationship and we represent them
with ovals like this and we put the name
of the attribute inside the oval shape
and then we connect them to whatever
entity or attribute they are an
attribute of and here is an example of
tying the old diagram together ok great
so in this diagram we have the user
entity on the left
having the attributes of first name last
name and email and we have the course
entity on the right having the
attributes of title description and
category and then the rest of that
diagram we are already covered
besides cardinality I think the other
common point of confusion that that
newbie A's encounter is you know when is
something at
attribute of an entity and when is it an
entity of its own well let's take a look
at that title attribute of the course
entity as an example the way that I
think if the title is is that it's just
text you know so somewhere in unity
systems any course has a simple string
of text associated with it that is
called its title there just isn't
anything more to know about the title of
a course than the title itself and for
that reason I consider it to be an
attribute on the other hand let's say
that we realize that well courses also
have lectures associated with them we
also know that lectures have a lot of
attributes themselves such as a title
and a time duration and probably other
stuff well we also know that lectures
are watched by users right so that's
that's a new relationship that we have
to think about all this complex
description means that lectures should
be an entity in their own right and by
the way we would diagram them like this
all right so you'll notice that we added
a lecture entity in the top right has
the attributes title and duration the
lecture entity has a one-to-many has
relationship with course meaning that a
course can have a end number of lectures
but a lecture can only belong to one
course and it also has a many to many
watches relationship with the entity
user so that means that M users can
watch end lectures so any number of
users can watch any number of lectures
that's pretty much the the ER diagram
it's funny I actually used to hate these
but just in the course of lecturing
about them I've I've come to love them
again so as you aged you'll be a career
you'll probably experience the same kind
of thing yourself but I hope that you'll
love the ER diagrams as much as I do