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According to book: FUNDAMENTALS OF Database Systems SEVENTH EDITION by Ramez & Navathe. In Chapter 9 which is related to converting EER to relational diagram there is a topic named: mapping of specialization or generalization to relational model and there are mentioned different rules from which one of the rule 8B is Multiple relations — subclass relations only. In which for a specialization whose sub classes are total(every entity in the super class must belong to (at least) one of the sub classes.

How can we create DDL of this type of relational model. Here we are using Vehicle_id as a primary key in both tables. Why we are not creating VEHICLE table and use its primary key as foreign key in CAR and TRUCK table.

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tyb9900
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  • The picture says to do exactly what you say you are not to do, so why do you think you're not supposed to do it & what do you think you're supposed to do? Also your first paragraph is unintelligible. Also "8B" in what presentation? Quote the presentation so we can address it. PS "Why not" is a question to ask the designers of the method. PS Please [use text, not images/links, for text--including tables & ERDs](https://meta.stackoverflow.com/q/285551/3404097). Use images only for what cannot be expressed as text or to augment text. Include a legend/key & explanation with an image. – philipxy May 08 '20 at 02:54
  • @philipxy, I updated my question and add some more details in first paragraph. Basically in question I want to clarify my confusion, according to book we need to create only sub class tables like mentioned in the picture but how can we implement this where the primary key is same for both tables. – tyb9900 May 08 '20 at 06:54
  • Please act on all my comment. Including, give the content of that image using text & explain exactly what it has to do with your textbook & your question. Look at the image. Your textbook will explain what the ERD notation says. It turns out that it says what the PK of vehicle is & it says what the PKs of the 2 subtypes are. What is your problem? Quote your textbook re EER & ERD & explain about being stuck the 1st place you are stuck. Don't ask us to rewrite your textbook. PS This is unclear. Use enough words, sentences & references to parts of examples to clearly & fully say what you mean. – philipxy May 08 '20 at 22:58

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