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databaseentityrelationshipentity-relationshipdata-modeling

Can anyone explain these notations from Erwin model?

This is ERD, and this is an actual model (most likely to be from Erwin). Can anybody explain notations I marked blue / red respectively?

Blue : As Dependents have total participation / can have many Employees, shouldn’t it just have | and ⩛? Why is there O?

Red : As Employees don’t need to participate in Policy relationship, shouldn’t it just have only O? Why is there |?

I thought what should be expressed on a line between an entity and a relationship is...

on an entity side : participation constraint (partial, total) is marked

on a relationship side : key constraint (1 to many... etc) is marked.

If I was wrong, I would appreciate if anyone can clarify this.

Solution

  • For the model you linked, while I can't explain why it's like that (I didn't design it), I can at least tell you what is being conveyed:

    • A Dependent can have zero or more records in the Policy table, and each record in the Policy table is related to exactly one record in the Dependent table. The relationship between Dependent and Policy is identifying (both because the PK in Policy contains all the FK columns from Dependent, and also because the relationship line between them is a solid line)
    • A Policy can be associated with at most 1 Employee, and an Employee can have at most 1 Policy. The relationship is non-identifying, both because the FK from Employee is below the line (not part of the PK) on Policy, and because the relationship line is dashed.

    The model looks like it might be from ERwin, but I'm not so sure about the ERD. The ERD might be in Korth notation, but it still doesn't look quite right... As it is, the arrow from Dependent to Policy seems backwards. If it is denoting cardinality, it should point the other way. I am assuming this is meant to be a many-to-one relationship. In that case, I think it would be correct if you swap the line from Employee to Policy with the arrow from Dependent to Policy, with the arrow going from Policy and pointing to Employee.

    Is this meant to be a locical/physical model split? That's my guess, and the logical modeler got their arrows mixed up for the relationship.