Aggregation is the process of compiling
information on an object, there by abstracting a higher level object. One
limitation of E-R model is that it cannot express relationship with in
relationship to illustrate the need for such a construct the ternary (compose of three) relationship work_on,
between employee branch job so in figure. Thus the relationship set work_on
relating the entity set, such an entity set is treated in the same manner as in
any other entity set. We can then created a binary relationship manager to
represent who manage what task
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