Erd assumptions

Erd assumptions

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We got an official scenario from Coventry University, and it is asking to Draw the ER diagram based on that scenario.

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The problem is; no one of the students got the right answer not satisfied with the ERDeven instructors and teachers were confused about it.

I answered it as what i have understand, however i am not satisfied with my answer. Consider the following scenario modeling project management activities in an organization. A car sales company offers to its customers the option of purchasing a car using repayment schemes of over 1 year, 2 years, 3 years, 4 years or 5 years.

A customer may purchase one or more cars under the scheme. When a customer purchases a car, he or she gets to choose one of the repayment scheme options for each purchase or may pay the whole amount in one go, meaning a repayment scheme option may be selected by one or more customers or may not be selected at all. A customer will have a unique customer number, a name, an address, and a phone number.

3. Logical Design

A repayment scheme option will have a unique repayment scheme number, a name and the number of years for repayment. A car will have a unique car code, a make, a model, year of manufacture and a price. The company pays commission on sales of cars to its salespeople and therefore it will also keep information about each sale which will include a unique sales ID and commission to be paid on that sale.

The company will also want to know who made the sales and the car involved in that transaction so that the commission can be determined. A salesperson will have a unique salesperson ID, a name, a phone number and an email address. Identify all the entities, attributes of each entity including primary key, foreign key, relationship between the entities and cardinality constraints. State any assumptions necessary to support your design. You're having some difficulty with keys.

None of your foreign keys need to be included in the primary keys. I read the assignment as saying he can choose a scheme per purchase:.

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When a customer purchases a car, he or she gets to choose one of the repayment scheme options for each purchase. By associating each car with a customer and salesperson, a car can only be sold once, and there's no relation between a sale and the car sold. You also didn't distinguish between entities and relationships.

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Foreign key constraints are not relationships, relationships in ER are implemented as tables or relationship relations in ER terminology, as opposed to entity relations. Foreign key constraints are just integrity constraints to make sure you don't relate non-existent values. I used Chen's notation since it distinguishes between entities and relationships. Foreign key constraints are represented by the lines between entities and relationships.

As you can see, this format doesn't indicate which tables represent entities or relationships, though it can be determined by looking at the primary keys of the tables. A somewhat more efficient design can be obtained by denormalizing relations with the same determinant:. Learn more. Confusing scenario to draw an ER diagram Ask Question.

Asked 3 years, 11 months ago. Active 27 days ago. Viewed 9k times. This is the scenario: Consider the following scenario modeling project management activities in an organization.

My answer: I want to know if my answer is right or not? Thank you. Khaledtaj8 Khaledtaj8 45 1 1 silver badge 7 7 bronze badges.Hence initially it can be null. Appointment Type states the type of appointment made that is the appointment is made by the patient through call, through direct visit or by a general physician.

Treatment Status in the Treatment Records entity helps to understand the result of the treatment type surgery or test whether it is positive or negative in case of test and success or failure in case of surgery.

Each patient can have more than one mode mode types include medicines, surgeries or allergies-At a time one Mode can be only of these which is identified using the IsAllergy,Is Current Medicine or Surgery columns.

Patients can pre-book appointments to the specialists in CQMC either by a phone call or by a direct visit to the medical center. Even the general physicians can book appointment to the specialist consults for any particular patient. While booking an appointment, the patient details are stored in the system and each patient is uniquely identified using the PatientID.

One general physician can refer one patient to a particular specialist any number of times. While booking appointments, patients past track of surgeries or any allergies and even the current medicines if any along with the dosage is stored. Upon consult, the specialist may conduct some tests and based on the problem can prescribe some medicines or even surgery.

Each item in the treatment phase carries different fee and the consultation fee is mandatory for any consult. Some specialists will offer some discounts to the patients based on the some predefined conditions. Assignment help Australia. My Assignment Help Australia. Cheap Assignment Help Australia. Online Assignment Help Australia. Professional Assignment Help.Thanks, I think it's a decent refresher course for those who have missed DB normalization class during good old college days :- Mike Kow.

Or for those of use whose college days were in the 70's and they didn't do ER diagrams on the punched cards we used! Nice article. One thing bothers me though Or am I missing something?

I would argue, however, that an ERD is simpler and better designed for a non-technical audience. I always walk my customers through an ERD prior to moving from analysis to design, and they usually learn how to read it and spot mistakes within minutes.

However, I will research this more and will post on it next. Nice article Lee! I did notice however, that you cast the ER diagram in the light of a class hierarchy, when I find the most suitable use for an ER diagram is when doing database schema design. Also, aren't relationship-entities such as in a many to many drawn as a diamond square turned 45 degrees?

As pointed out by the above anonymous coward, class hierarchies are usually shown using UML notation, but UML is usually not used for database schemas unless you begin "overloading" the UML notations to bind their database equivalents e.

The main difference between the two are that ERs are best for detailing structured data, whereas UML has notations for behaviors methods in addition to data, making them more suitable for modeling class hierarchies who have both state and behavior. It might also be good to detail the other "edge notations" such as Chen notation, using 1:N, M:N, etc. Scotty Z, Thanks for your post man, I'll have to break my response up. This is simply good normalization. And also just to get all stakeholders using the same language.

But if you've done your job right, the class diagrams and ERD's should be nearly identical anyway.

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Regardless I've never seen anyone use the diamond, but I could see how it would be helpful. I should have cited Peter Chen to begin with even though he doesn't use the crow's feet notation that I like so much. Like they always say, "A picture is worth a thousands words. Thanks for putting this article together. I came across it looking for a refresher on interpreting ER diagrams and this did the trick. Eric H Miami of Ohio '94 Xavier ' I thought one-to-many relationships were enforced through the use of foreign key constraints Am I missing something?

But that's the beauty of ERD's. They abstract away implementation details and just focus on the important parts: Entities and their relationships. And in the process removes numerous arguments like surrogate vs. Thanks dude!! Many thanks, needed some info on how many-to-many relationships are actually implemented.All database systems are based on some assumptions.

These assumptions are only determined to be correct or faulty once the database is built and tested. Faulty assumptions do not always create a negative impact on the database; they actually assist the database developer in finding ways to improve the project design and functions of the database.

Some developers assume that each table of a database only has one column primary key field that automatically gets incremented as more data is inputted. This is faulty because it is allowable to override this assumption manually.

erd assumptions

The downside is the developer has to hand code additional parameters to make the database accept multiple columns containing other primary keys. Some developers assume that a JPanel system is limited to only edit a single table in the database. This is not true because the JPanel code design allows two or more generated panels to merge into one panel, but information must be cut and pasted by the developer. Additional hand coding is also necessary to implement the merging of data.

These assumption are introduced to be special case assumptions because they support the basic idea of "database completion" while simultaneously satisfying the instinctive elements of the database process. Studies prove that the syntactic and semantic definitions used for CWAs are equivalent and therefore can be considered as viable assumptions. Semantic assumptions are temporary data that are saved in an unstable database for the purpose of deriving inherent information from the solidified data in the system.

There are two types of semantic assumptions: point-based and interval-based. The point-based assumption relates to the usage of interpolation methodologies, while the interval-based assumption generally deals with various property types, such as time coarseness.

Joey Liam is a freelance writer specializing in sports, politics, entertainment and technology. He started his professional writing career in Liam studied information systems at National University. Written by: Joey Liam Written on: February 04, References PSU. About the Author.An entity relationship diagram ERD shows the relationships of entity sets stored in a database.

An entity in this context is an object, a component of data. An entity set is a collection of similar entities. These entities can have attributes that define its properties.

By defining the entities, their attributes, and showing the relationships between them, an ER diagram illustrates the logical structure of databases.

There are two reasons to create a database diagram. You're either designing a new schema or you need to document your existing structure. If you have an existing database you need to to document, you create a database diagram using data directly from your database.

erd assumptions

You can export your database structure as a CSV file there are some scripts on how to this herethen have a program generate the ERD automatically. This will be the most accurate potrait of your database and will require no drawing on your part.

erd assumptions

If you want to create a new plan, you can also edit the generated diagram and collaborate with your team on what changes to make. Peter Chen developed ERDs in An ER diagram is a means of visualizing how the information a system produces is related. There are five main components of an ERD:. When documenting a system or process, looking at the system in multiple ways increases the understanding of that system.

ERD diagrams are commonly used in conjunction with a data flow diagram to display the contents of a data store. They help us to visualize how data is connected in a general way, and are particularly useful for constructing a relational database. Learn More. ER diagrams are used to sketch out the design of a database.

Get started making database designs Sign up for SmartDraw free. Works on your Mac or any other device. Hospital Billing Entity Relationship Diagram. Banking Transaction Entity Relationship Diagram. Follow Us.An entity—relationship model or ER model describes interrelated things of interest in a specific domain of knowledge.

A basic ER model is composed of entity types which classify the things of interest and specifies relationships that can exist between entities instances of those entity types. In software engineeringan ER model is commonly formed to represent things a business needs to remember in order to perform business processes. Consequently, the ER model becomes an abstract data modelthat defines a data or information structure which can be implemented in a databasetypically a relational database.

Entity—relationship modeling was developed for database and design by Peter Chen and published in a paper. An E-R model is usually the result of systematic analysis to define and describe what is important to processes in an area of a business. It does not define the business processes; it only presents a business data schema in graphical form.


It is usually drawn in a graphical form as boxes entities that are connected by lines relationships which express the associations and dependencies between entities. An ER model can also be expressed in a verbal form, for example: one building may be divided into zero or more apartments, but one apartment can only be located in one building.

Entities may be characterized not only by relationships, but also by additional properties attributeswhich include identifiers called "primary keys". Diagrams created to represent attributes as well as entities and relationships may be called entity-attribute-relationship diagrams, rather than entity—relationship models.

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An ER model is typically implemented as a database. In a simple relational database implementation, each row of a table represents one instance of an entity type, and each field in a table represents an attribute type. In a relational database a relationship between entities is implemented by storing the primary key of one entity as a pointer or "foreign key" in the table of another entity. Note that the conceptual-logical-physical hierarchy below is used in other kinds of specification, and is different from the three schema approach to software engineering.

The first stage of information system design uses these models during the requirements analysis to describe information needs or the type of information that is to be stored in a database. The data modeling technique can be used to describe any ontology i. In the case of the design of an information system that is based on a database, the conceptual data model is, at a later stage usually called logical designmapped to a logical data modelsuch as the relational model ; this in turn is mapped to a physical model during physical design.

Note that sometimes, both of these phases are referred to as "physical design. An entity may be defined as a thing capable of an independent existence that can be uniquely identified. An entity is an abstraction from the complexities of a domain. When we speak of an entity, we normally speak of some aspect of the real world that can be distinguished from other aspects of the real world.

An entity is a thing that exists either physically or logically. An entity may be a physical object such as a house or a car they exist physicallyan event such as a house sale or a car service, or a concept such as a customer transaction or order they exist logically—as a concept. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type.

An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type.Earlier in this chapter, we showed you how to design a database and understand an Entity Relationship ER diagram. This section explains the requirements for our three example databases— musicuniversityand flight —and shows you their Entity Relationship diagrams:.

The music database is designed to store details of a music collection, including the albums in the collection, the artists who made them, the tracks on the albums, and when each track was last played. The university database captures the details of students, courses, and grades for a university.

The flight database stores an airline timetable of flight routes, times, and the plane types.

Entity-relationship modelling

The next section explains these databases, each with its ER diagram and an explanation of the motivation for its design. The music database stores details of a personal music library, and could be used to manage your MP3, CD, or vinyl collection. It ignores the requirements of many music genres, making it most useful for storing popular music and less useful for storing jazz or classical music.

When a track is played, the date and time the playback began to the nearest second should be recorded; this is used for reporting when a track was last played, as well as the number of times music by an artist, from an album, or a track has been played. Conversely, each play is associated with one track, a track is on one album, and an album is by one artist. The attributes are straightforward: artists, albums, and tracks have names, as well as identifiers to uniquely identify each entity.

The track entity has a time attribute to store the duration, and the played entity has a timestamp to store when the track was played. If you wanted to use the music database in practice, then you might consider adding the following features:.

Support for compilations or various-artists albums, where each track may be by a different artist and may then have its own associated album-like details such as a recording date and time. Under this model, the album would be a strong entity, with many-to-many relationships between artists and albums. Playlists, a user-controlled collection of tracks. For example, you might create a playlist of your favorite tracks from an artist.

Source details, such as when you bought an album, what media it came on, how much you paid, and so on.

erd assumptions

Album details, such as when and where it was recorded, the producer and label, the band members or sidemen who played on the album, and even its artwork.

Smarter track management, such as modeling that allows the same track to appear on many albums. The university database stores details about university students, courses, the semester a student took a particular course and his mark and grade if he completed itand what degree program each student is enrolled in. We explain the requirements next and discuss their shortcomings at the end of this section.

A program has a name, a program identifier, the total credit points required to graduate, and the year it commenced. A course has a name, a course identifier, a credit point value, and the year it commenced.

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Students have one or more given names, a surname, a student identifier, a date of birth, and the year they first enrolled. When a student takes a course, the year and semester he attempted it are recorded. When he finishes the course, a grade such as A or B and a mark such as 60 percent are recorded.

Each course in a program is sequenced into a year for example, year 1 and a semester for example, semester 1. Although it is compact, the diagram uses some advanced features, including relationships that have attributes and two many-to-many relationships. Each student must be enrolled in a program, so the Student entity participates totally in the many-to-one EnrollsIn relationship with Program.

A program can exist without having any enrolled students, so it participates partially in this relationship. As a weak entity, Course participates totally in the many-to-one identifying relationship with its owning Program.

This relationship has Year and Semester attributes that identify its sequence position. Student and Course are related through the many-to-many Attempts relationships; a course can exist without a student, and a student can be enrolled without attempting any courses, so the participation is not total. When a student attempts a course, there are attributes to capture the Year and Semesterand the Mark and Grade. Our database design is rather simple, but this is because the requirements are simple.

For a real university, many more aspects would need to be captured by the database.

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