When modeling a relationship you have to think about what that relationship really means. The cardinality is an important part of this meaning.
Below is an explanation of the different cardinalities.
Cardinality |
Symbol |
Description |
One-to-one |
1..1 |
One instance of an entity always corresponds to another instance of a second entity. |
One-to-many |
1..n |
One instance of an entity corresponds to one or more instances of a second entity. |
Many-to-one |
n..1 |
More than one instance of an entity can correspond to the same single instance of a second entity. |
Many-to-many |
n..n |
More than one instance of an entity can correspond to more than one instance of a second entity. |
There's a groupbox for each of these types at the top of the Relationship Properties dialog. But there is more, a relationship can have additional properties:
Property |
Description |
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Dominant role |
In a one-to-one relationship, this will determine the direction of the relationship. |
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Role name |
Description the relationship of the first entity to the second entity and vice versa. |
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Required |
Indicates that each instance of a first entity requires an instance of the second entity. This influences the cardinality as described below. |
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Cardinality |
In addition to the groupboxes, you can change the cardinality by either using the Required checkbox, or selecting an entry from the drop down box.
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Identifying relationship |
For identifying relationships, the master identifying attributes become part of the detail identifying attributes. An identifying relationship is a continues line, while a non-identifying relationship is a dashed line. |
The Dominant Role, Cardinality and Required attributes influence the behaviour when generating a physical data model.
In the diagram, the end points of each relationship show you how the relationship is defined:
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Many, but optional |
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One, but optional |
|
Many, requires at least 1 |
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One, required |
See also: