The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered a good option for designing databases. Here is some discussion upon connectivity.
Entity Relation Model for Computer choice and many to much relationship is resolved from the refined ER Model below to design database solution for Computer Choice.
E-R Diagram for Computer Choice Show how identified entities are related to each other and what are the attributes of each Entity
The connectivity of a relationship describes the mapping of associated entity instances in the relationship. The values of connectivity are "one" or A one-to-one (1:1) relationship is when at most one instance of a entity A is associated with one instance of entity B.
For example, "employees in the company are each assigned their own office. For each employee there exists a unique office and for each office there exists a unique employee.
A one-to-one (1:1) relationship is when at most one instance of a entity A is associated with one instance of entity B. For example, "employees in the company are each assigned their own office. For each employee there exists a unique office and for each office there exists a unique employee.
A one-to-many (1:N) relationships is when for one instance of entity A, there are zero, one, or many instances of entity B, but for one instance of entity B, there is only one instance of entity A. An example of a 1:N relationships is A department has many employees
Each employee is assigned to one department
A many-to-many (M:N) relationship, sometimes called non-specific, is when for one instance of entity A, there are zero, one, or many instances of entity B and for one instance of entity B there are zero, one, or many instances of entity A.
An example is: employees can be assigned to no more than two projects at the same time; projects must have assigned at least three employees
A single employee can be assigned to many projects; conversely, a single project can have assigned to it many employee. Here the cardinality for the relationship between employees and projects is two and the cardinality between project and employee is three.
Many-to-many relationships cannot be directly translated to relational tables but instead must be transformed into two or more one-to-many relationships using associative entities.
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