RelationalDBDesignRelationalDBDesign





ER Diagrams   «Prev 

Illustrating one-to-one Relationships

Connectivity of a Relationship

The connectivity of a relationship describes a constraint on the connection of the associated entity occurrences in the relationship. Values for connectivity are either
  1. one or
  2. many.
For a relationship between the entities Department and Employee, a connectivity of one for Department and many for Employee means that there is at most one entity occurrence of Department associated with many occurrences of Employee. The actual count of elements associated with the connectivity is called the cardinality of the relationship connectivity; it is used much less frequently than the connectivity constraint because the actual values are usually variable across instances of relationships. Note that there are no standard terms for the connectivity concept, so the reader is admonished to consider the definition of these terms carefully when using a particular database design methodology.


1:1 relationship between Table A (entity A) and Table B (entity B) from the viewpoint of Table A.
1:1 relationship between Table A (entity A) and Table B (entity B) from the viewpoint of Table A. Note that one record in Table A relates to one record in Table B (A:B = 1:1)

The same relationship between Tables A and B appears this way from the viewpoint of Table B. Note that one record in Table B relates to one record in Table A( B:A = 1:1)
The same relationship between Tables A and B appears this way from the viewpoint of Table B.
Note that one record in Table B relates to one record in Table A (B:A = 1:1)