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For example, many students belong
Computer Science through Entity-Relationship Diagrams (d) A many-to-many relationship is
to the same class.
(ERD). ERD uses basic graphical symbols
to show the organisation of entities and
when one entity from set A can
their relationships. An important aspect
be associated with more than one
of relationships is cardinality which
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defines how the entities’ data or records
For example, students are related to
are related. The common cardinalities entity from set B and vice versa.
include one-to-one, one-to-many, many- multiple teachers, and teachers can
to-many, and many-to-one relationships. be associated with multiple students.
Details for each relationship are outlined
as follows: Cardinality notation symbols
(a) A one-to-one relationship is when In cardinality notation symbols, the lines
one entity from entity set A can be connecting entities appear with extra
associated with at most one entity marks at the end. These marks define
the cardinality degree. The notations are
of entity set B and vice versa. For summarised as s:hown in Figure 8.15.
example, a school might decide to
assign one class teacher to precisely One
one class; thus, one teacher manages
only one class. Many
(b) A one-to-many relationship is when One and only one
one entity from entity set A can be
associated with multiple entities of Zero or one
entity set B. Still, an entity from entity
set B can be associated with at least One or many
one entity in set A. For example, the Zero or many
relationship between a teacher and
the subjects they teach, where one Figure 8.15: Cardinality notation
teacher can teach multiple subjects,
but only one teacher teaches a given Generally, these cardinalities might
subject. apply in an ERD at the entity level. An
example is the customer order database
(c) A many-to-one relationship is when in Figure 8.16, in which a salesperson
more than one entity from entity set serves many customers and each can
A can be associated with at most also place multiple orders. Each order
one entity of entity set B. However, can have only one invoice, but each of
an entity from entity set B may or these orders may also include many line
may not be associated with more items, and each specifies the products
than one entity from entity set A. that it contains.

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