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Lesson 3 | Types of Relationships |
Objective | Describe the three types of Relationships |
Three Types of Relationships in ERD Diagram
There are three types of relationships that can exist between two entities.
An entity-relationship [ER] diagram can be created based on these
three types, which are listed below:
- one-to-one relationship: In relational database design, a one-to-one [1:1] relationship exists when zero or one instance of entity A can be associated with zero or one instance of entity B, and zero or one instance of entity B can be associated with zero or one instance of entity A. [abbreviated 1:1]
- one-to-many relationship: [abbreviated 1:N] In relational database design, a one-to-many [1:N] relationship exists when, for one instance of entity A, there exists zero, one, or many instances of entity B; but for one instance of entity B, there exists zero or one instance of entity A.
- many-to-many relationship: In relational database design, a many-to-many [M:N] relationship exists when, for one instance of entity A, there exists zero, one, or many instances of entity B; and for one instance of entity B, there exists zero, one, or many instances of entity A. [abbreviated M:N]
1:1 relationship | In a traditional American marriage, a man can be married to only one woman; a woman can be married to only one man. |
1:N relationship | A child has exactly one biological father; a father can have many biological children. |
M:N relationship | A student can enroll in many classes; a class can have many enrolled students. |
In the business world, one-to-one relationships are few and far between. One-to-many and many-to-many relationships, on the other hand, are common. However, as will be explained later, many-to-many relationships are not permitted in a relational database and must be converted into one-to-many relationships. Relational databases are comprised almost entirely of tables in one-to-many relationships.
Types of Constraints
Limit the number of possible combinations of entities that may participate in a relationship set. There are two types of constraints:
- cardinality ratio and
- participation constraints.
Very useful concept in describing binary relationship types. For binary relationships, the cardinality ratio must be one of the following types:
1] One To One
An employee can work in at most one department, and a department can have at most one employee.2] One To Many
An employee can work in many departments [>=0], but a department can have at most one employee.3] Many To One
An employee can work in at most one department [=0], and a department can have several employeesThe following page contains three diagrams describing the 3 relationship types implemented in Microsoft Access.
Three Relationships in MS Access.
The next lesson defines one-to-one relationships.
Relational Database Design