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Editorial Team
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Asked: 2026-06-07T16:38:49+00:00

I’m doing a project for my Database exam, and I’m stuck at a tricky

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I’m doing a project for my Database exam, and I’m stuck at a tricky point. This is an excerpt of my ER diagram:

https://i.stack.imgur.com/d1WQd.png

As you can see, the entity Employee is generalization of Waiter and Cook. Head waiter and Chief cook are specializations of respectively Waiter and Cook. Only chiefs can fullfill an Order (non included in this excerpt) to the supplier. Obiously only one Chief at a time can Fulfil an order (I’ve made a single Fulfilment relationship to make the diagram less messy).

After remodeling the ER diagram (sorry if I’m not using the correct words, I’m translating in English), this is the result:

https://i.stack.imgur.com/GFOyD.png

So an Employee must have a Role (Waiter, Cook or whatever else), and for each role there could (obviously) be only one Chief. I really don’t know if this is correct, the double generalization confuses me a lot.

Next step is the logical diagram (tables), and this is a total mess!
This is a possible solution:

enter image description here

This is a possible translation to the relational model (with an Unique Not Null constraint on FK2 in tblChief) , but it has a weakness: if you change Employees role in tblEmployee, tblChief won’t see this error and integrity will be broken. Could adding a FK between tblEmployee(Role) and tblChief(Role) be a good solution? Or would it be ugly/messy?

I’ve thought of a second possible solution. I prefer it, but still I don’t know if it’s OK:

enter image description here

Here I’ve merged the entity Role with the entity Chief in a single table. The field Chief references IDEmployee, again it must be Unique and Not Null.

Are these solutions correct? I’m sure that this will be a big mess when inserting and updating Employees and Roles (AFAIK it can’t be done without temporarily removing the integrity checks). Could be this done in a different way?

Sorry for the long post (and for the image links, but I can’t post them directly) and many thanks if you’ll answer me 🙂

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1 Answer

  1. Editorial Team

    The last diagram does not make sense.

    Each employee has 1 role (this is a FK relationship)

    Each role can have one superior role. This is a nullable FK relationship back to the role table. (Itself). Not to employee as you have it.

    To find all employees that are managers you would have a select statement like this:

    SELECT * 
FROM employees
WHERE roleID in (SELECT roleID 
                 FROM role 
                 WHERE superiorID IS NULL)

    or

    SELECT * 
FROM employees E
LEFT JOIN role R ON E.roleID = R.roleID
WHERE R.superiorID IS NULL
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