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ER

My question is very basic. In following diagram, why we are required to en-box entity CENTER, relationship OFFER, and entity COURSE? Why we don't just place a box on relationship OFFER.

If I am not wrong, then Centers which do not offer Courses OR Courses which do not have Centers are not included in aggregation.

So, why do we include those two entities in box?

Aggregation deals with relationship alone, so, can't we place aggregation box on diamond i.e. relationship OFFER.

I am fairly new to DBMS (I have just read 2 chapters of Raghu Ramakrishnan 2nd edition). I am bit confused about the Aggregation.

Ubi.B
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1 Answers1

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You can put the box on the diamond alone - in fact, I prefer that notation. See for example When to use Associative entities? and What is the right way to use Associative Entity?

reaanb
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    But there is a difference between aggregation and associative relationship? – Ubi.B Oct 30 '19 at 10:11
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    I am confuse please reply – Ubi.B Oct 30 '19 at 10:22
  • The term I used is "associative entity", not "associative relationship". All relationships are associative. The concept of an associative entity is that we want to define a relationship on a relationship, so we "objectify" the latter relationship, treating as if it is an entity. I don't like the term aggregation, but it's just another term for the same idea in ER modeling. – reaanb Oct 30 '19 at 10:56