The above ER
diagram has cardinality ratio n : m (many-many) because in 1 project many employees can
work and 1 employee can work in many projects.
Note: In this diagram, we see that the relationship works on
has an attribute workTime. In case of n-m relationships, the diamond
relationship can have attributes if such attributes cannot be directly assigned
to the entities. For example, if the entity employee is assigned workTime, we
cannot find out how much time the employee works for which project. On the
other hand, if we assign workTime to Project entity, we cannot find out which
employee works for how much time for a particular project. It is further to be
noted that in case of binary relationships 1:1 and 1: n, the middle diamond
relationship cannot have any attributes.
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