Database management system Lecture 7 : Strong and weak entity sets

ER diagram with cardinality limits
• E-R diagrams also provide a way to indicate more
complex constraints on the number of times each
entity participates in relationships in a relationship
set.
• An edge between an entity set and a binary
relationship set can have an associated minimum and
maximum cardinality, shown in the form m..n, where
m is the minimum and n the maximum cardinality.
Dinesh Kumar Bhawnani, BIT DURG

• A minimum value of 0 indicates partial participation of the
entity set in the relationship set.
• A minimum value of m indicates total participation of the
entity set in the relationship set.
• A maximum value of m indicates that the entity participates
in at most one relationship, while a maximum value *
indicates no limit.
• Note that a label m..* on an edge is equivalent to a double
line.

CUSTOMER LOANborrower
Customer_Id
Customer_Name Customer_Street
Customer_City
Loan_Number Amount

Weak and Strong Entity Sets
• An entity set may not have sufficient attributes to form a primary
key. Such an entity set is termed a weak entity set.
• An entity set that has a primary key is termed a strong entity set.
• For e.g., consider the entity set payment, which has the three
attributes: payment-number, payment-date, and payment-
amount. Payment numbers are typically sequential numbers,
starting from 1, generated separately for each loan. Thus,
although each payment entity is distinct, payments for different
loans may share the same payment number. Thus, this entity set
does not have a primary key; it is a weak entity set.

L1, 1 MILLION
L2, 2 MILLION
L3, 2 MILLION
-
-
-
-
1,10000
2, 10000
3, 10000
1, 50000
2, 50000
LOAN Loan-payment PAYMENT

• For a weak entity set to be meaningful, it must be
associated with another entity set, called the
identifying or owner entity set.
• Every weak entity must be associated with an
identifying entity; that is, the weak entity set is said to
be existence dependent on the identifying entity set.
• The identifying entity set is said to own the weak
entity set that it identifies.

• The relationship associating the weak entity set with the
identifying entity set is called the identifying relationship.
• The identifying relationship is many to one from the weak
entity set to the identifying entity set, and the participation
of the weak entity set in the relationship is total.
• In our example, the identifying entity set for payment is loan,
and a relationship loan-payment that associates payment
entities with their corresponding loan entities is the
identifying relationship.

• Although a weak entity set does not have a primary key, we
nevertheless need a means of distinguishing among all those
entities in the weak entity set that depend on one particular
strong entity.
• The discriminator of a weak entity set is a set of attributes
that allows this distinction to be made.
• For e.g., the discriminator of the weak entity set payment is
the attribute payment-number, since, for each loan, a
payment number uniquely identifies one single payment for
that loan.

• The discriminator of a weak entity set is also called the
partial key of the entity set.
• The primary key of a weak entity set is formed by the
primary key of the identifying entity set, plus the weak entity
set’s discriminator.
• In the case of the entity set payment, its primary key is {loan-
number, payment-number}, where loan-number is the
primary key of the identifying entity set, namely loan, and
payment-number distinguishes payment entities within the
same loan.

• The identifying relationship set should have no descriptive attributes, since
any required attributes can be associated with the weak entity set.
• A weak entity set can participate in relationships other than the identifying
relationship.
• A weak entity set may participate as owner in an identifying relationship
with another weak entity set.
• It is also possible to have a weak entity set with more than one identifying
entity set. A particular weak entity would then be identified by a
combination of entities, one from each identifying entity set.
• The primary key of the weak entity set would consist of the union of the
primary keys of the identifying entity sets, plus the discriminator of the
weak entity set.

GATE Question
• Which of the following is used to represent the supporting many-one
relationships of a weak entity set in an entity-relationship diagram?
A. Diamonds with double/ bold border
B. Rectangles with double/ bold border
C. Ovals with double/ bold border
D. Ovals that contain underlined identifiers
[GATE 2020 CS/ IT, IIT DELHI]

Steps in ER Modelling
•Usually the following five steps are followed to
generate ER models
1.Identify the entity set.
2.Identify the relevant attributes.
3.Identify the prime attribute.
4.Find relationships between entity set.
5.Draw a complete ER model.

Question 1
• Draw an ER Model for an University database application
where
a) A University has many departments.
b) Each department has multiple instructors; one among
them is the head of the department.
c) An instructor belongs to only one department.
d) Each department offers multiple courses, each of which
is taught by a single instructor.
e) A student may enroll for many courses offered by
different departments.

Step 1: Identify the entity set
• From the given question, we can identify the following entity
sets.
1. DEPARTMENT
2. COURSE
3. INSTRUCTOR
4. STUDENT
• “Head of the department” is NOT an entity set; it is
relationship between the INSTRUCTOR and DEPARTMENT
entities.

Step 2: Identify the relevant attributes
• For the DEPARTMENT entity set the relevant attributes
are Dept_No, Dept_Name and Location.
• For the COURSE entity set the relevant attributes are
Course_No, Course_Name, Duration, and Pre-requisite.
• For the INSTRUCTOR entity set the relevant attributes are
Inst_Id, Inst_Name, Room_No, and Telephone_No.
• For the STUDENT entity set the relevant attributes are
Student_No, Student_Name, and Dob.

Step 3: Identify the Prime (key) attribute
•Dept_No is the key attribute for DEPARTMENT
entity set.
•Course_No is the key attribute for COURSE entity
set.
•Inst_Id is the key attribute for INSTRUCTOR
entity set.
•Student_No is the key attribute for STUDENT
entity set.

Step 4: Identify the relationship between
entity sets
1. Each department has multiple instructors and an instructor
belongs to only one department.
DEPARTMENT INSTRUCTORhas

entity sets
2. Each department has multiple instructors; one among
them is the head of the department.
DEPARTMENT INSTRUCTORheadedby

entity sets
3. Each department offers multiple courses.
DEPARTMENT COURSEoffers

entity sets
4. Each department offers multiple courses, each of which is
taught by a single instructor.
INSTRUCTOR COURSEtaughtby

entity sets
5. A student may enroll for many courses offered by different
departments
STUDENT COURSEenroll

Step 5: Draw the complete E R Diagram
STUDENT
DEPARTMENT
INSTRUCTORCOURSE
Inst_Id
Inst_Name
Room_No
Telephone_No
Stud_No
Stud_Name
Dob
Course_Name
Course_No
Duration
Dept_No Dept_Name
Location
Pre-requisite
enroll
has
headedoffers
taughtby

University Questions
1. Write short notes on

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Database management system Lecture 7 : Strong and weak entity sets

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