ER diagrams also are often used in conjunction with data flow diagrams (DFDs), which map out the flow of information for processes or systems. Rules for DFD's: Entity Relationship Diagram Information Systems Computer Science. Here you can see an example of the DFD, which shows the overall data flow for making a ERD means the Entity-relationship Diagram, which reflects the key.
Start your own creations easily right now by checking more related stylish and personalized resources from Edraw.
Organizations use such models to effectively communicate within different divisions. This article will explain the differences between DFD and ERD with examples, but first, let us check out the basic definitions of these two models.
DFD is generally used for outlining the pattern and framework of a data system without showing processing time options in sequence, for example, the Yes or No choices in typical flowcharts. In reality, DFD can be used for information management or data visualization. Here you can see an example of the DFD, which shows the overall data flow for making a reservation in a restaurant.
The most common practical uses of ERD include relational databases in software engineering, academic institutions and so on. Below is a good example of the ERD for school information management. In this case, a single entity unit can be teacher or student.
Additionally, all the personal details of teachers and students are in green. Data Flow Diagram for Hotel Application For the hotel application, the DFD needs to show the reservation state changes after it passes through a hotel process. This is the same reservation state shown in the use case pre- and post-conditions. This is the same reservation state shown in the use case pre- and post-conditions and the data flow diagram.
State Diagram for Reservation For the hotel application, the State Machine Model needs to show the sequence of the valid values for the reservation state open, confirmed, fulfilled, cancelled, and closed. This is the same reservation state shown in the use case pre- and post-conditions, the data flow diagram and the entity relationship diagram.
Summary The state attribute is the logic glue that holds these models together.
What is Entity Relationship Diagram (ERD)?
The analyst needs to verify the models for consistency and completeness Figure 5. Use cases model the functions that set states. Have them classify their example s.
Obtain a data model from a source other than the textbook. Ask the students to classify the entities. Attributes Attribute — a descriptive property or characteristic of an entity. Synonyms include element, property, and field. Just as a physical student can have attributes, such as hair color, height, etc. Synonyms in different data modeling languages are numerous: Go back to the slide showing the sample ERD Figure Pick an entity and ask the students to list attributes that they feel describe those entities.
Show the students a form.
Ask the students to identify the attributes. Be sure that the students recognize what items appearing on the form are truly attributes and those that are simply headings or preprinted items.
Also, often students accidentally identify attribute values as attributes.System Design (DFD and ER diagrams)
For example, they may say that an item that appears as a check box is an attribute when in fact it may be the value of an attribute ie. Identification Key — an attribute, or a group of attributes, that assumes a unique value for each entity instance. It is sometimes called an identifier.
Concatenated key - group of attributes that uniquely identifies an instance. Candidate key — one of a number of keys that may serve as the primary key. Primary key — a candidate key used to uniquely identify a single entity instance. Alternate key — a candidate key not selected to become the primary key.
Teaching Notes Students can generally relate to the following example. Suppose you are working for an hourly wage. The employer has some method of tracking the hours you work. Whether that involves a time clock, an identification badge that it scanned, or a log book, the system records a certain number of hours and some employee identifier that says those hours are yours.
Without that identifier, come pay day the employer would not know whose hours were whose. The employer might pay someone else for the hours you worked. Relationships Relationship — a natural business association that exists between one or more entities. The relationship may represent an event that links the entities or merely a logical affinity that exists between the entities. Teaching Notes Explain that there may be more than one relationship between two entities.
Cardinality Cardinality — the minimum and maximum number of occurrences of one entity that may be related to a single occurrence of the other entity. Because all relationships are bidirectional, cardinality must be defined in both directions for every relationship.
Teaching Notes Ask the students to read or write declarative sentences to reflect the bidirectional meaning of the relationship between student and curriculum.
Degree Degree — the number of entities that participate in the relationship. A relationship between two entities is called a binary relationship. A relationship between three entities is called a 3-ary or ternary relationship. A relationship between different instances of the same entity is called a recursive relationship.
Provide the students with an ERD that does not contain relationships. Ask the students to identify possible relationships and indicate a possible degree for that relationship. Emphasize to the students that the degree represents a business rule! Failure to accurately identify and document the degree will result in a system that does not reflect a correct business requirement. Logical models reduce the risk of missing business requirements because we are too preoccupied with technical results.
Logical models allow us to communicate with end-users in nontechnical or less technical languages. Flow of data through processes Logic Policies Procedures Data flow diagram DFD — a process model used to depict the flow of data through a system and the work or processing performed by the system. Synonyms are bubble chart, transformation graph, and process model. The DFD has also become a popular tool for business process redesign.
Teaching Notes Many, if not most students have drawn or seen process models in the form of program flowcharts. Unfortunately, flowcharts are control-flow process models as opposed to data flow process models. This can cause some students trouble because they want to illustrate structured flow of control nonparallel processing in their early DFDs.
Most introductory information systems books at least introduce, with one or two examples, DFDs. Describe the classes in the system, and the static relationships between classes. Class diagrams are used during Analysis, Design and Development.
Class Diagrams Class diagrams that appear during the Analysis phase generally describe user interface and related classes. In UML Distilled read the description of class diagram perspectives located on page Most of the class diagrams will be generated during Design. Some may be generated during early Development, and classes will often be modified during Development. Static relationships are relationships between classes which can be thought of as object templates.
Customer Rental Invoice Rental Item In general, associations describe the relationships between classes or objects.
Rules for DFDs:Entity Relationship Diagram Information Systems Computer Science
They start as simple connectors, to show that some relationship exists. As design progress the natures of the relationships become clearer, and the associations are modified to reflect the new knowledge. For example, direction and cardinality are usually added later in design. The diagram above shows a generalization relationship between several kinds of objects. Generalization means here that the specializations of Rentable Item are fully consistent with Rentable Item and may be used wherever Rentable Item is allowed.
All parts but the Name are optional.
Explain the Differences Between DFD and ERD with Examples
The Parts of a Class An individual class is shown on the diagram as a box with up to four sections. The first section shows the name of the class. Names should be nouns. Typically the first letter of every word in a name is capitalized. Attributes represent the state of the object or what the object knows.