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Entity-Relationship (ER) Diagram — Transcript

by Neso Academy · 2,833 words · 421 segments · language en · Watch on YouTube

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0:01[Music]
0:06hello everyone welcome back the most
0:09awaited topic of this chapter The Entity
0:12relationship diagram which is also
0:15called as simply ER diagram let's step
0:18into the topics of the day what we are
0:20going to learn in this presentation we
0:23are going to understand the entity
0:25relationship diagram which is the ER
0:27diagram and also we are going to
0:29understand this ER diagram with the help
0:31of the basic structure and components of
0:34the ER model while waiting let's step
0:37into the topic of the day the ER diagram
0:40as I mentioned ER diagram means it is
0:43the entity relationship diagram we know
0:45what is an entity and we know how
0:47relationship is also formed among
0:49entities so this diagram is going to
0:52basically involve the entities and the
0:54relationship among the entities and we
0:57also have seen the basic components of
0:59the ER model which which are the entity
1:00sets the attributes and the relationship
1:03sets let's start with the basics of this
1:06ER diagram now basically this ER diagram
1:09is used to express the overall logical
1:12structure of a database we have already
1:14seen about the ER model which is mainly
1:16used for expressing the overall logical
1:19structure of a database and we know ER
1:21model is a high level data model and
1:24basically this ER diagram is a graphical
1:27representation so it is depicted or
1:30portrayed as diagrams so er model when
1:33it is portrayed as diagrams they are
1:35represented as ER diagrams and that is
1:38why it is mentioned that this is a
1:40graphical representation and as I
1:42mentioned a picture is a worth of
1:44thousand words so it's easy to
1:47understand things with the picture and
1:49that to when it involves a lot of
1:50complexities instead of having a written
1:53or a verbal communication if things are
1:56represented with the help of diagrams it
1:58has more advantages and hence these are
2:02simple and clear while looking at the
2:04diagram itself we will be able to
2:06understand how many relations are there
2:08I mean how many tables are there what
2:10are the attributes in the table how
2:12relationships are formed among the
2:14entities which is a primary key
2:15attribute is there any weak relationship
2:18Etc and because of its Simplicity and
2:21the very clear nature of depicting the
2:23things it has a widespread usage in the
2:26database deployment areas we are done
2:29with the basics of ER diagram let's step
2:32into the notations that are used in the
2:34ER diagram when we talk about entities
2:37The Entity set is actually represented
2:39using rectangle and if it is a single
2:41rectangle it means it is a strong entity
2:44set and when things are represented with
2:46double rectangle then it is a weak
2:48entity set don't worry about this what
2:51is a strong entity set and what is a
2:52weak entity set now anyway I'm going to
2:55show you some examples shortly that time
2:57I will explain you what is strong entity
3:00set and what is weak entity set with
3:02some examples so this is about the
3:04entity set and what about the
3:06relationship when we have strong entity
3:09set and weak entity set we also do have
3:12strong relationship and weak
3:14relationship when relationship is
3:16expressed with a single Diamond it is
3:18strong relationship if it is depicted
3:21with two diamonds or a Double Diamond it
3:24is a weak relationship anyway when we
3:27understand strong entity and weak entity
3:29that time I will explain you what is
3:31strong relationship and what is weak
3:33relationship just recollect what are the
3:35basic components of ER diagram or ER
3:37model The Entity sets the relationship
3:41what is missing attributes and these
3:43attributes in ER diagram are represented
3:46like this we have already seen about
3:47this in the previous presentation single
3:49oval it is simple attribute or single
3:52valued attribute and composite attribute
3:54is represented like this multivalued
3:57attribute is represented with double
3:58oval and derived attribute is expressed
4:01with dotted oval and remember we also
4:04have null values in the attribute level
4:06so these are the notations that are used
4:09in the ER diagram in order to understand
4:11things clearly let's see few examples
4:14and things will be easy for you to
4:16understand let me show you a part of the
4:18ER diagram here it is this instructor is
4:23an entity student is an entity
4:26instructor and student have advice is
4:28relationship so in instructor advises
4:31student and what are the attributes of
4:33instructor relation the instructor ID
4:35name and salary what are the attributes
4:38of student entity student ID name and
4:41credits earned so this is how actually
4:44an ER diagram is represented now when
4:47you look into this it is a simple
4:50rectangle and it is a simple diamond
4:52symbol so it means this is a strong
4:54entity and this is also a strong entity
4:57and if we represent with double I mean
4:59double rectangle or Double Diamond then
5:02it is a weak entity or weak relationship
5:04let's see that shortly there are some
5:07conventions that represents ER diagram
5:09like this but in this lecture series I'm
5:12going to represent the same kind of ER
5:14diagram like this can you see here I'm
5:16going to represent entity and attributes
5:18like this so the first half is the
5:20instructor which is the name of the
5:22entity set like here and the attributes
5:25are represented like this ID name and
5:27salary which are the attributes ID name
5:29salary here and the same advises
5:32relationship here and the equivalent for
5:34this entity student is represented like
5:36this student with the attributes ID name
5:39and credits can you see here ID name and
5:41credits and again this is a strong
5:43relationship only because it is
5:45represented with single Diamond if it is
5:48Double Diamond then it is a weak
5:49relationship now you may be wondering
5:51what is a strong relationship and what
5:53is a weak relationship let me explain
5:55that now let me take the same a diagram
5:58and go to a new page in order to depict
6:00this so let me focus on only this part
6:04now when you look into this this is an
6:06ER diagram that has only two relations
6:08or two tables or two entities to be
6:11precise instructor entity and student
6:14entity and what is the relationship it
6:16is advises relationship I mean
6:19instructor advises student I'm going to
6:23bring in something from the previous
6:25lecture can you recollect this we have
6:28seen the basic structure and components
6:30where the relationship among the
6:32entities are depicted if you are
6:35directly watching this lecture I request
6:37you to watch my previous lecture I mean
6:39part two of the ER model in order to
6:41understand things better this is the
6:43entity set instructor and this is the
6:45entity set student and we have advises
6:48relationship and the same is here the
6:50instructor entity set student entity set
6:53and advises relationship now what is
6:55additional here the date attribute that
6:58gives more information about the
7:00relationship when the relationship is
7:02formed I mean on what date this
7:05instructor is assigned as the adviser
7:07for this particular student let me take
7:10John John is appointed as the instructor
7:13for Jennifer on third of May 2024 now
7:17there is an attribute here and what
7:19attribute is this it's a date attribute
7:22that gives more information about this
7:24relationship isn't it how this is
7:26represented in ER diagram we are going
7:28to use dotted lines with the attribute
7:32date in order to tell the relationship
7:34attribute what is this relationship and
7:37what attribute it is date attribute that
7:40tells or gives more information about
7:42the association when the instructor is
7:45assigned as the adviser to the student
7:48so what is this diagram this is the
7:51attribute attached to the relationship
7:53set what is the relationship set the
7:56advises is the relationship set that has
7:59an attribute that is attached to this
8:01relationship set isn't it so we use
8:04dotted lines in order to represent this
8:06remember what is a dotted line it is an
8:09attribute attached to a relationship set
8:12what is this it's a line line means it
8:16is used to connect the entity with the
8:17relationship set or the relationship set
8:20with the entity so two entities are
8:23associated with the help of relationship
8:25set and lines are used to connect the
8:27entity with the relationship set so
8:30remember two types of lines are here the
8:32dotted line and the straight line so
8:35dotted lines are to connect the
8:37attribute with the relationship set so
8:39attribute attached to the relationship
8:41set is represented using dotted line and
8:44straight line is used to connect the
8:46entity or entity set with the
8:47relationship let's see another example
8:50and what we are going to see now the
8:52strong entity set if you see the
8:55relationship is with single Diamond so
8:57obviously this is a strong relationship
8:59and at the same time if you see here
9:01this is also single rectangle only so
9:04this is a strong entity set and this is
9:06also strong entity set now we need to
9:08understand what is a strong entity set
9:11when an entity has a primary key
9:13attribute then it is a strong entity set
9:16because this entity set is not depending
9:19on any other entity because it has a
9:23primary key attribute which helps in
9:25identifying the records uniquely
9:27similarly coming to this entity set
9:29student we also have a primary key
9:31attribute here so the relationship that
9:34is formed between two strong entity set
9:37is going to be a strong relationship
9:39only and that is why this relationship
9:41is a strong relationship I hope now you
9:43are able to understand what is a strong
9:45entity set the entities in the strong
9:48entity set are not dependent on other
9:50entity also it will have a primary key
9:54attribute let's now see the weak entity
9:56set here is an example for weak entity
10:00can you see here it is represented using
10:02Double Diamond so when it is represented
10:04with Double Diamond it is a weak
10:07relationship and also what are new
10:09things here this is a strong entity set
10:12but this is a weak entity set the reason
10:14is there is no primary key attribute and
10:17the attributes are underlined with
10:19dashed lines so from this it's clear
10:21that there is no primary key attribute
10:24like this attribute in this course
10:25relation however in section there is no
10:28primary key attribute since there is no
10:30primary key attribute this is a weak
10:32entity set so the relationship formed
10:35between a strong entity set and this
10:37weak entity set is a weak
10:39relationship also we have double light
10:42what does it mean this is dealing with
10:44the participation whether it is a total
10:46participation or an optional
10:48participation or partial participation
10:51don't worry about this now we have an
10:53exclusive lecture that deals with
10:55participation so for now just understand
10:58double lines deals with with total
11:00participation the entities present in
11:02the entity set must compulsorily
11:04participate in at least one entity in
11:07the relationship anyway I will explain
11:09more about this later also we have an
11:12arrowed line here there are some
11:14conventions that uses arrowed line to
11:17represent the cardinality ratio or the
11:19cardinality number but in this lecture
11:21series we are going to represent
11:23something different because here the
11:25exact numbers cannot be specified anyway
11:27in the coming presentation you are going
11:29to understand about the mapping
11:30cardinalities in diagram for now just
11:33understand this is a weak relationship
11:35This Is the participation related and
11:38this is cardinality related so this
11:40example is for a weak entity set because
11:44this entity set has no primary key
11:46attribute also the attributes are
11:48dependent on a strong entity set in
11:50order to ensure its existence when we
11:54see an example for ER diagram we will be
11:56able to know things better since we are
11:58going to deal about the participation in
12:00the coming presentation I'm going to
12:02ignore this for now what about this this
12:05is actually the cardinality ratio isn't
12:07it so here instructor and student are
12:11having a relationship instructor advises
12:13student and what is the cardinality
12:16ratio this is represented with some
12:18numbers here so no worries this is the
12:21way to represent the cardinality limits
12:23in the next presentation I will tell you
12:25the numbers here and how it differs when
12:27we give numbers here and here
12:30so this is how cardinality limits are
12:32represented in the ER diagram let's see
12:35the next example here is an example of
12:38an instructor entity this instructor
12:41entity has ID name address phone number
12:46date of birth age and salary as the
12:49attributes I request you to pause this
12:51video for a while and compare this with
12:54the type of attribute that we have seen
12:56in the recent lecture
12:59I hope you are done from this we can say
13:03this is a primary key attribute this is
13:05a composite attribute right so name has
13:08first name middle name and last name
13:10what about the attribute address this is
13:12a complex attribute because it has
13:15Street and Street has Street number
13:17street name and apartment number and
13:20what are the other attributes in the
13:21street it's city state and zip zip or
13:25postal code so remember this is a
13:27composite attribute under another
13:29composite attribute and what about this
13:31phone number we have seen this phone
13:33number is a multivalued attribute so
13:35multivalued attributes are represented
13:37using this flow brackets and what about
13:40date of birth this is a single valued
13:43attribute this is simply mentioned just
13:44like that single valued attribute or
13:47even simple attributes what about this
13:50age this age is a derived attribute from
13:53which attribute this age can be derived
13:55from this date of birth attribute and
13:57that is why age is represented with
13:59parenthesis meaning it is a derived
14:02attribute what about salary this is a
14:05simple attribute because this is atomic
14:07in nature which cannot be decomposed
14:09into further components or further
14:11values so salary is a simple attribute
14:14which is just like that represented so
14:17what we understand from here is this is
14:19how an entity is represented with
14:21different types of attribute now I want
14:24to show one more thing here which is
14:26this we have only one entity
14:29which is course which has course ID as
14:32the primary key attribute which has
14:33other attributes like title and credits
14:36and we also have a relationship and this
14:39relationship is a strong relationship
14:41only and the relationship is
14:43prerequisite is it a binary relationship
14:46no a binary relationship is formed
14:49between two entity sets but here we have
14:51only one entity and we have this
14:53relationship achieved with the help of
14:56course ID and prerequisite ID then what
14:59kind of relationship is it it is
15:01recursive relationship I will elaborate
15:04about this recursive relationship in the
15:06a diagram examples but for now I will
15:08just give you only few things let's say
15:10there are two courses here course one
15:12and course two let's say course two that
15:15requires course one as a prerequisite
15:17let's say if one student has to undergo
15:19course two then course one should have
15:22been completed in that case course one
15:25and course two are interl so that course
15:27two can be complet
15:29if and only if course one is completed
15:32and that is why these kind of
15:34relationship are represented as
15:35recursive relationship I'm not
15:37articulating more about this because we
15:39are going to see that when we see some
15:41examples in the ER diagram now let me
15:44show you another example just tell me is
15:47it an attribute or an entity it is an
15:50entity right if it is an attribute we
15:53have already seen in this presentation
15:55it is connected with a dotted line so
15:57that this is a descriptive attribute we
15:59have seen date as an example but here
16:01this is Project which is an entity set
16:03not an attribute like instructor like
16:06student and these three are strong
16:09entities and that is why we have a
16:11strong relationship now what this
16:13example is all about this is an example
16:17for tary relationship because this
16:19relationship is formed between three
16:21entities instructor project and student
16:25remember the third entity is connected
16:28with the help of dotted lines because
16:30this exist only when these two exist
16:33remember these dotted lines or dashed
16:35lines are used to link attributes of a
16:38relation to the relationship set or it
16:40is used in a tary relationship and
16:42that's it guys what are the topics we
16:44have seen in this presentation we have
16:46seen the entity relationship diagram
16:48which is the ER diagram with various
16:50notations that are used in the ER
16:52diagram and also we have understood the
16:54basic structure and components of the ER
16:56model with the help of some examples in
16:59E
17:00diagram I hope the session is
17:02informative and thank you for watching
17:06[Applause]
17:08[Music]

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