I have recently learned about “barred relationships” when developing an ERD through the Oracle Academy service. The course gave the following definition of a barred relationship: “A relationship that participates in an entity’s unique identifier” I understand how this works in the context of resolving a M:M relationship by creating an intersection entity and having the UID of that entity come from the originating relationships. My question is: Can a barred relationship occur in an instance where there is no intersection entity, or are barred relationships reserved for intersection entities?
Barred, that is, bar-ed ids are literally the original ids of the two entities connected by the relation with a bar (pipe, |) in between. Like in a CSV file.
The term is unusual and an invention of that course, I would say. It certainly is not usually used in the generic SQL world, nor even in the Oracle RDBMS documentation.
Related
I feel like I have searched through the internet to find an answer to this question for quite some time now, but without success. Does anyone feel comfortable explaining why many-to-many relationships should be replaced with a bridge table?
Probably most (all??) RDMS implement a M:N relationship by creating a table containing two columns with the FKs.
So there is no advantage to explicitely model the bridge table.
But in most realistic cases you want to store additional information (besides the fact of its existence) about the relationship instance, e.g. timestamp and user from the creation. That means that you need to model the bridge table anyway.
Quick question about an ER/EER Diagram.
I have made this Entity Relationship Diagram, but I have been told, that there is something wrong with it by a friend. Is there something wrong with it?
The ER diagram is a design of a Library Management System, where a member can borrow 5 books at a time. The rest of the functionality of the system is how a normal library functions.
Library Management System EER
i don't understand the utility of the relationship between the librarian and the card and i don't understand why the books are splitted in two entities.
I would do 3 entities:
-member
-card
-book
every member has one card, every card is of one member;
every member can take many books, every book can be taken by many members,
the relation between member and book create another table in the logic schema: loans. before inserting a new loan you can check if the member has alredy 5 active loans (by checking the attribute active in the loans table).
Your given context is incomplete for me. I do not see the whole description of your problem/situation, so I will answer based on assumptions, and the experience I had during my life. So let's see...
The tino user questioned the existence of two entities, title and volume, which is something important. Let me explain this for a moment, which will eliminate this as an error. Previously (a time ago) we had video rental stores (I don't know if this the right name where you live, english is not my native language). Remember? We used to go there to rent VHS tapes to watch at home.
What we rented were not films, but more copies/midia of them. A film will always have the same actor, director, title, etc., but a copy could have different attributes/properties, like the year that the media was manufactured, the available languages, the expiration year, among other things. So we had distinctly two different things.
But despite this, we have to consider whether there is a need to create two entities for persistence. We have to remember if we need to persist this information. If a copy/midia has no attributes, then it's entity should not exist, and what a user would rent really would be the movies titles.
In your case, the relationship between volume and title, I belive, is really expressing this discrepancy.
Let's talk about the relationship between librarian and title. What a librarian manages? Does It manages the titles that never change and are abstract things, or the physical objects present in the library? :)
Finally, let's talk about the borrows relationship. When we break down 1-N (or N-1) relationships, we always pass the primary key from the 1 side to the N side, solving the relationship to the formation of the Physical Model in a Entity-Relationship Diagram.
Despite this relationship here is a 0-5, to decompose it, we will not have exactly a 0-5 relationship. We would have in anyway to pass the primary key from the two sides to the table formed by this relationship. Therefore, here we have initially a N-N relationship between member and volume.
N-N relationships allow optional relations between entities. This means we can have the zero side cardinality here. To limit the number of books that can be rented, you need to implement a restriction/constraint with SQL, or with any procedural language in your database. In this case, you can implement a before insert trigger. This trigger has a duty to verify this restriction to allow or denny the completion of the operation as a whole.
Let it be clear that I'm not saying you should remove this notation. Your Conceptual Model should express it. But when you are decomposing, you have to remember that. I think you should just correct it.
Remember one important rule: Relations that have attributes/properties (the attributes/properties) can only exist in N-N relationships. If you have to put attributes/properties in a 1-N (or a N-1) relation, they (the attributes/properties) will always be on the N side. In summary, there are no N-1 (or 1-N) relationships with attributes in the relation. Only N-N relations can have attributes/properties. So be careful with this.
Any questions or clarification, please comment and I will answer.
I see no reason to distinguish member and card. Volume and Librarian don't have primary keys. Are they supposed to be weak entities? That doesn't make sense for Librarian and Volume needs an identifier to distinguish different copies.
I have some questions need to be answered. The requirement is to build a database for university entrance system:
"Applicant may apply to 5 university, each university may or may not interview the applicant, then, make an offer to the applicant. The offer may or may not be conditional (conditional/uncondtional), if the offer is conditional, the conditions are stored. The applicant need to choose which conditional offers he/she wishes to accept, up to maximum of 3. If any of the conditions is met when end of the year, the offer becomes unconditional, then, applicant may accept that one."
There are some notable points:
The course work requires using some enhanced ER features such as supertype/subtype.
Regardless the offer is conditional or unconditional, the applicant can accept the offer. Am I right?
In my ERDs, the APPLICATION entity is a weak entity, using a surrogate key, and UNIQUE_CONSTRAINT on University_ID and Applicant_ID.
In my ERD (on working), there are 2 versions. The ERD_1 is a suggestion of my friends. But I think, my work on ERD_2 is more accurate. I have questions:
Am I correct when use a surrogate in the APPLICATION entity? OR using composite of University_ID and Applicant_ID is a primary key?
Could APPLICATION entity be an associate entity? If it is, it could have some subtypes?
In ERD_2, how to show the ACCEPT relationship between APPLICANT and OFFER entity? AND how to show the MAKE relationship between UNIVERSITY and OFFER?
ERD_1
ERD_2
I would appreciate any help.
I can think of no reason why a weak entity could not be analyzed into subtypes (aka subclasses aka specializations). However, your two ERDs suggest that your analysis of your case is not one of specializations. In particular, in your first ERD, you use the word "has" to describe the relationship between an application and an interview or an offer.
"Has-a" relationships are typically not generalization-specialization relationships. "Is-a" relationships typically are. Example: An auto is a vehicle and a truck is a vehicle.
There is an entirely different issue here when it comes to what tables, columns, and constraints you will use to implement your model. That's a design issue rather than an analysis issue.
I don't understand your case well enough to agree or disagree with your analysis of your case.
I want to store information about song files which they are categorized by:
1. genre for example: pop, rock, classical etc.
2. instrument used like piano violinn etc
3. Artist
Each song has many instruments. Each song has many artist.
So all of the relations are many-to-many. How can I implement this? Is it good idea to make many-to-many relation between song entity and those three category type entities? or should I only have a single category entity that inherits those sub categories?
thanks in advance
The instrument and the artist are both examples of "has-a" relationships. The genre is an example of an "is-a" relationship. Has-a and Is-a relationships are modeled quite differently in relational database design. A song is a pop song, or a song is a rock song. (I presume that a song can be both pop and rock, but that's not clear in your description).
Has-a relationships are more common, and are generally covered in any good tutorial. Is-a relationships are often not given enough coverage in those same tutorials. If you want to get some articles about ER modeling of is-a relationships, lookup "generalization/specialization". This will explain how to depict one of these relationships but not how to design tables to fit.
If you are familiar with object modeling, your easiest handle on the concept is known as class/subclass pattern aka type/subtype. This is straightforward, but it depends on built in support for inheritance. This support is generally present in any object system, and does most of the hard work for you. If you are designing a relational database you come face to face with the fact that the relational model does not directly support inheritance. This means that you have to design tables that mimic the benefits of inheritance, unless you are willing to use SQL extensions available in your dialect of SQL.
Two relatively standard designs are known by the names of these two tags:
single-table-inheritance
class-table-inheritance
You can get a brief description by reading the Info tab of each of these tags. You can get an even better description by looking up Martin Fowler's treatment of these two patterns.
The single table solution will help you create a single table for all song files, with columns that pertain to any or all of the categories.
The class table solution will help you design one table for the song files and one table for each genre. It will be helpful if you use the shared primary key technique and thereby cause each entry in a genre table to "inherit" its ID from the corresponding entry in the song file table. You have to make this inheritance happen by extra programming when you go to insert a new song file.
Which one is better? It depends on your case. You decide.
My assignment requires that I have an ER diagram that shows the dependencies between the objects in my solution. Mine consists of several procedures, tables, a trigger and a sequence. Does anyone know if these objects are permitted in a SQL ER diagram? I ask this because the example shown by my lecturer consisted only of tables. If these objects are allowed in an ER diagram, what is the proper way to represent them?
No, an ER diagram should not contain procedures, triggers nor sequences.
An Entity-Relashioship Diagram is used to represent the relashionships between entities in a database. Procedures, triggers and sequences do not contribute to the relashionship representations.
ER Diagram don't have any particular representation of triggers because ER-Diagram is logical design of Database. BUT we use Strong entity and weak entity symbols to show the dependence of tablesStrong Weak
You should use Entities (the squares), Relationships (the diamonds) and attributes (the ellipsis). All of them are connected by simple lines.
Finally, and most important. Entities and Relationships are not Tables, as you say. Those are part of a physical model.