Recursive relationship

Recursive relationship: An entity can be related to itself

 

The above ER diagram comprises of products consisting of a hierarchy of assemblies. One assembly contains other assemblies. One assembly is contained in other assemblies.

1-1-1 ternary relationship

1-1-1 ternary relationship (one-to-one-to-one ternary relationship)

The above ER diagram has cardinality ratio 1:1:1. Let us assume 1 department has 1 manager and 1 secretary. Now 1 department with 1 manager works with 1 secretary. 1 department with 1 secretary works with 1 manager. 1 manager together with 1 secretary works in 1 department.

 

Many-to-many-to-many ternary relationship

n-m-k ternary relationship (many-to-many-to-many ternary relationship)

In the above ER diagram, the cardinality ratio is n:m:k. One supplier can supply for one project many parts. The same part can be supplied by one supplier for many projects. The same part can be supplied for one project by many suppliers.

Note: In all ternary relationships, the middle diamond relationship can have attribute(s) when those attribute(s) cannot be directly assigned to the entities as has been explained  in case of many-many binary relationships.