I've been asked by my teacher to make the ER diagram of a certain library. Here is a very simple and not complete ER diagram only to illustrate my point:
Searching the library's website I found exactly the attributes I need for the customer, for the copies the book etc. However, the book is not the only thing provided by the library. The library provides music-CDs , journals, DVDs and many more leading to 20 different kinds of items in total.
One very ugly solution would be to create a different entity for all these items and connect each one of them with the copy entity. And this seems unavoidable since each item type has different attributes. A book for example has ISBN and a journal the ISSN. Looking at all the attributes of each item the only thing they all have in common is a title.
Is there a better way to do the ER diagram?
unfortunately no, he is asking for an academic solution and the way to represent the mini-world in er is 1 entity type per object. this is my vision to ER diagrams in its simplest academic form.
if you continue to sum entity types together, your entire mini world will be one entity type.
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I usually find everything I need, but there is first time for everything.. :D
I am trying to create a conceptual data model and I dont know how to properly show M:N relationship, which by default should not be included, but still you can assign verbs and directions of abstract entities. So let's say we have "Projects" and we have "Project roles", how do I properly show relationships? Can I have 2 arrows as showed in the picture or do I have to add join table and ?? I can't wrap my head around this..
Thank you so much in advance :)
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A conceptual data (or information) model can be created with a suitable modeling language, such as ER diagrams or UML class diagrams. Both languages have a concept and a visual notation for many-to-many associations (or relationship types). Simply follow their definitions. Since there is no standard for ER diagrams, it's easier/preferable to go with UML.
For showing a many-to-many association between two classes (representing entity types), you draw a connection line and annotate it with an asterisk ("*") at both ends.
Notice that a join table is a database implementation, and not a modeling concept.
I'm working on a classified ads site with 12 categories. E.g. category vehicles has items cars, bikes, Commercial Vehicles and spare parts. The following is a flow diagram for posting an ad:
I need to show the specification in the Form Filled section of the above image to the users in dropdown lists in the form when they are posting an advertisement. The car specification will be its color,engine,fuel type.
The ERD is below :
How should this issue be tackled, what are the best practices and is the current design going along the right lines?
On the whole this looks ok. Here are some observations:
likes.iker_id should point at users.id? Just trying to understand your model to start.
I would probably change the pics table to be one pic per row and then add an ordinal for ordering.
One question here is how you intend to look at your graph model. As it is, you might have a graph that could be traversed easily to a depth, a couple deep. I assume you are doing this to recommend ads. If so, I think this is sufficient. If not it would be good to further discuss which rdbms you are targetting.
Hope this helps:
In a simplified case, you will need some extra tables.
So, you are trying to be able to have different specifications for different items in your categories? Or, in other words, it is like having different attributes for different types of products in an e-commerce website.
If that the problem you are tackling, then you should look into the Entity–Attribute–Value (EAV) model that is how the problem is solved. By the way, one of the most popular open source e-commerce engines uses it as well.
i agree look at EAV models...
for some other tables, you have many normalization issues - for example:
you should have a separate address table (not part of the ad)
you should have a picture table (and link those to the ads with another table)
you should have a person table - and link that to the ad as 'owner'
the idea of 'favorite' should also be in this person->ad relationship table as a role or type column
I'm trying to refactor some parts of a legacy database schema and am having trouble with coming up with the correct design.
The entities in question are:
samples, papers, studies
papers are associated with many samples
studies are associated with many samples
papers and studies have their own attributes not compatible with each other
samples can be associated with multiple papers and multiple studies
However, this separates out the grouping of papers and studies.
Here's how it looks:
An alternative I thought of was since both papers and studies are just grouping the samples together, I can combine these as one, and have FK from the group into their respective paper/study table.
Here's how it looks:
I'd like to know if the designs look reasonable and if there are any tradeoffs between the two different designs? Also are there alternatives to modelling the relations?
I think the first design is a right one. There are two M:M relations, Paper - Sample and Study - Sample. They are different by domain logic, so there is no sense to combine them in one relation and introduce extra entities for that purpose. First schema is a good normalized one. What is your goal? What problems do you try to resolve?
the schema doesn't have explicit grouping ...
OK, if you do require Group as a separate entity, your design could look like this:
The problem is, Group entity is weak. It is hard to propose any attribute to this entity except for ID. It is not handy to work with this scheme thought. When user edits paper's group, you have to choose, how to handle this situation. Should all other papers\studies 'see' this change too, or you have to create\search edited group and assign it to paper. I think it is wrong way to take if there is no additional business logic related to groups. Usually, when weak entities appear in a design, it means that set of abstractions has been chosen not properly. At the moment, I don't see how to justify Group entity.
This is probably a simple problem for an experienced database developer, but I'm struggling... I have trouble translating a certain ER diagram to a DB model, any help is appreciated.
I have a setup similar to slide 17 of this presentation:
http://www.cbe.wwu.edu/misclasses/mis421s04/presentations/supersubtype.ppt
Slide 17 shows an ER diagram with an Employee supertype having an Employee Type attribute and as subtypes the Employee Types themselves (Hourly, Salaried and Consultant), which is very similar to my design situation.
In my case, suppose Salaried Employees are the only ones that can be bosses of other employees and I wanted to somehow indicate if a certain Salaried employee is the boss of the Hourly and/or Salaried Employee and/or Consultant (either, none or both), how could that be designed in a database model, also considering these are one-to-many relationships?
I can put a PK-FK relationship between them, which would result in all tables having two FKeys and (like Consultant having FK_Employee and FK_SalariedEmployee) and SalariedEmployee referencing itself, but I keep thinking that might not be the wisest solution....although I'm not sure why (integrity issues?).
Is this or an acceptable solution or is there a better one?
Thanks in advance for any help!
Your case looks like an instance of the design pattern known as “Generalization Specialization” (Gen-Spec for short). The gen-spec pattern is familiar to object oriented programmers. It’s covered in tutorials when teaching about inheritance and subclasses.
The design of SQL tables that implement the gen-spec pattern can be a little tricky. Database design tutorials often gloss over this topic. But it comes up again and again in practice.
If you search the web on “generalization specialization relational modeling” you’ll find several useful articles that teach you how to do this. You’ll also be pointed to several times this topic has come up before in this forum.
The articles generally show you how to design a single table to capture all the generalized data and one specialized table for each subclass that will contain all the data specific to that subclass. The interesting part involves the primary key for the subclass tables. You won’t use the autonumber feature of the DBMS to populate the sub class primary key. Instead, you’ll program the application to propagate the primary key value obtained for the generalized table to the appropriate subclass table.
This creates a two way association between the generalized data and the specialized data. A simple view for each specialized subclass will collect generalized and specialized data together. It’s easy once you get the hang of it, and it performs fairly well.
In your specific case, declaring the "boss of" FK to reference the PK in the Salaried Employees table will be enough to do the trick. This will produce the two way association you want, and also prevent employees who are not salaried from being referenced as bosses.
I am relatively new to CakePHP, and I am writing a CakePHP application which currently has an Author model and a Book model. Author and Book both have a many-to-many relationship.
However, I would like to additionally, for every author-book relationship, have a corresponding link to that author's blog where they reflect upon their experiences writing that book.
If that was confusing, the following paragraphs try to elaborate to make my situation more clear:
For every author, there will be one article about their experiences for each book. To look at this relationship in reverse, this means that for every book, there will be one corresponding link, per author (given that books can have multiple authors), where that author describes his/her experience writing the book.
In other words, for every single (Author, Book) pair, I would like to be able to store a related URL.
In more fancy, abstracty math terms that I will use horribly and improperly: I have an undirected graph, where every vertex is an Author or a Book, and every edge has an Author for one vertex and a Book for the other. That sounds kind of confusing, but (I think) it describes my situation as precisely as I can make it.
Given the above description, I want to know if I can attach arbitrary data to any given edge of that relation, and retrieve it later.
Currently I have a third model, AuthorBook, which acts the graph edge.
Author has a hasMany relationship to AuthorBook.
Book has a hasMany relationship to AuthorBook.
AuthorBook has a belongsTo relationship with Author and Book, and additionally has a url field.
This solution works, but for some reason the fact that Book is not directly connected to Author is bothering me. Is there any way to achieve this same effect while also cutting the AuthorBook model out of the picture?
(Also, I realize I can have them directly related while keeping AuthorBook, but that seems ugly and also redundant.)
(Also, I realize that I would still have an author_book SQL table. This does not bother me in the slightest, I think it's impossible to do without anyway.)
Thank you for your time! And please don't flame me!!!
Yes, you can model the AuthorBook relation (or the 'edge' as you envision it).
class Author extends AppModel {
...
$hasAndBelongsToMany = array('Book', array('with' => 'AuthorBook'));
....
}
The "with" habtm option:
Defines the name of the model for the join table. By default CakePHP will auto-create a model for you. Using the example above it would be called RecipesTag. By using this key you can override this default name. The join table model can be used just like any "regular" model to access the join table directly.
This is the equivalent of RoR's "through", depicted quite nicely in this illustration