Entity Relationship Diagram Templates | Editable Online or Download for Free | Creately
A Entity Relationship Diagram showing computer hardware shop ER diagram. You can edit this Entity Relationship Diagram using Creately diagramming tool. Editable Entity Relationship Diagram templates to quickly edit and add to your presentations/documents. Many exporting options, styling options to quickly. Construct a clean and concise ER diagram for the NHL database. Answer: . Meaning and definition of case study. b). Purpose of case study. c) .. Notebook manufacturers are willing to supply as many notebooks as the.
On the first screen of the Reverse Engineer Wizard, do the following: Select the database driver for your database management system DBMS. Select the data source of the database that you are updating. If you have not already created a data source for the existing database, click New to do so now. When you create a new source, its name is added to the Data Sources list.
When you are satisfied with your settings, click Next. Follow the instructions in any driver-specific dialog boxes. For example, in the Connect Data Source dialog box, type the user name and password, and then click OK. If you use the ODBC Generic Driver, you may receive an error message that indicates that the reverse engineered information may be incomplete.
Select the check boxes for the type of information that you want to extract, and then click Next. Some items may be unavailable appear grayed out because not all database management systems support all the kinds of elements that the wizard can extract. Select the check boxes for the tables and views, if any that you want to extract, or click Select All to extract them all, and then click Next. If you are reverse engineering an Excel worksheet and don't see anything in this list, it is likely that you need to name the range of cells that contains the column headings in your worksheet.
If you selected the Stored Procedures check box, select the procedures that you want to extract, or click Select All to extract them all, and then click Next.
Entity Relationship Diagram Templates | Editable Online or Download for Free
Select whether you want the reverse engineered items to be added automatically to the current page. You can choose to have the wizard automatically create the drawing, in addition to listing the reverse engineered items in the Tables and Views window. If you decide not to have the drawing created automatically, you can drag the items from the Tables and Views window onto your drawing page to manually assemble the database model. Review your selections to verify that you are extracting the information that you want, and then click Finish.
The wizard extracts the selected information and displays notes about the extraction process in the Output window. This ability is limited to only VisioModeler 2. On the Database tab, in the Model group, click Import, and then click the model type.
Type the path and file name for the model that you want to import, or click the Browse button to locate the model file, and then click Open. In the Import dialog box, click OK. Visio imports the file and displays its progress in the Output window. The imported tables are displayed in the Tables and Views window. In the Tables and Views window, select the tables that you want to model, and then drag them onto the drawing page. After you create a database model diagram, the work of refining the diagram begins.
You can add and customize tables and views, create relationships, and customize columns and data types. Tables Use the Entity shape to create a table in your diagram. From either the Entity Relationship or Object Relational stencil, drag an Entity shape onto the drawing.
Double-click the shape to open the Database Properties window. Under Categories, click Definition and type a name for the table. Under Categories, click Columns, type a name, and choose a data type. Select the Req'd check box for columns that can't have null values. Select the PK primary key check box for columns that uniquely identify each row in the database table. Columns Use the Database Properties window to add or change properties for columns, including data types and primary keys.
Double-click the table in your diagram. In the Database Properties window, under Categories, click Columns. Click in the first empty Physical Name cell, and type a name. To change the data type for a column, click the column's Data Type field, and then select a data type from the list or type it into the list. For example, you can type decimal 8,2 or char Crow's foot diagrams represent entities as boxes, and relationships as lines between the boxes.
Different shapes at the ends of these lines represent the relative cardinality of the relationship. Crow's foot notation was used in the consultancy practice CACI. With this notation, relationships cannot have attributes.
Where necessary, relationships are promoted to entities in their own right: Model usability issues[ edit ] You can help by adding to it. February In using a modeled database, users can encounter two well known issues where the returned results mean something other than the results assumed by the query author. The first is the 'fan trap'. It occurs with a master table that links to multiple tables in a one-to-many relationship.
Create a Database Model (also known as Entity Relationship diagram)
The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses. When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results.
The solution is to either adjust the model or the SQL. This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue.
The second issue is a 'chasm trap'.
That possibility is just as viable as the possibility I have assumed. This is part of the attraction of this type of work. If you were actually creating a database in this example, you would have to ask someone what the situation actually is. But since you are just given this description, you have to come up with some assumption. In other words, for every one department there can be, at most, one managing employee. This information can also be represented in the ER diagram: As you might have determined, the M part of a relationship is represented by putting an M next to the appropriate entity type in the relationship while the 1 part is represented by a 1.
The ER diagram now represents much more information than it did above: Any one division can contain many departments. Any one department can be contained in, at most, one division.
Any department can have, at most, one managing employee or manager. Any manager can manage, at most, one department.
If you are a bit confused about all this 1: Several other questions remain about this situation that are not addressed in the description: What is the minimum number of departments in a division?
Does a department have to be associated with a division? Does a department have to have a manager? These questions would have to be answered before we complete the ER model. And we will answer these questions later. For now we are going to stop this part of the analysis since the purpose of this example is to demonstrate what ER modelling is all about. The ER modelling process is not something for which a set of steps can be given and then performed.
The process contains almost as much art as science.
Some steps are performed many times and many decisions are re-visited and revised. Given these conditions, a broad outline can be given: Determine what entity types are involved. Determine which entity types are related. Refine the definition of the relationships. Understand now that there are several methods for representing ER models graphically. Notice what has happened with this situation. Initially we had a text description of the problem.
After analysing it and making some necessary assumptions, we created an ER diagram that reflects the situation accurately and makes explicit the relationship among the entity types. This is why we perform ER modelling. It is quite a straight-forward step to go from this ER model to an implemented database.
Remember why we are doing all this: We are finding out all we need to know to create a database that will hold our data. And a well-defined database can be a very useful tool for solving business problemsand it is also in high demand by recruiters. You will learn how to perform the steps necessary to create such a database in later chapters.
Introduction to Entity-relationship modelling
In this section I present more detail on some of the basic concepts. In the example in an earlier section, we saw that divisions are directly associated with departments and departments are directly associated with employees.
No direct association between division and employee was given. This does not mean that there is no relationship between division and employee. In fact, the ER diagram tells us that there is a relationship between the two: Given any one division, there can be many employees managing departments within that division. Certainly, this is not earth shattering news. But it is in the ER diagram. The above fact is not represented as a separate relationship between division and employee because it can be inferred from existing relationships.
An ER diagram should contain the minimum number of relationships necessary to reflect the situation. For relationships between two entity types, there are three basic cardinalities.
Each of the following descriptions are given in terms of a relationship between entity type X and entity type Y. One entity of type Y can be associated with, at most, one entity of type X.
A car has only one steering wheel and a steering wheel can only be installed in one car. M one-to-many One entity of type X can be associated with many entities of type Y. A building can have many rooms but a room can be in, at most, one building. M many-to-many One entity of type X can be associated with many entities of type Y.
One entity of type Y can be associated with many entities of type X. A car can have many options and an option can be installed on many cars.Entity Relationship Diagram (ERD) Tutorial - Part 1
Determine the cardinality of the relationships between the following four pairs of entity types. For each relationship you have to answer two questions: Answering these two questions gives you the answer to the following questions. For example, if you answered M to the first question and 1 for the second question, then this relationship between entity types X and Y is of cardinality M: Patient under care of primary care physician Physician performs operation Doctors have speciality in disease Needle injected into patient It would seem that at any particular time a patient can only have one primary care physician and that any physician can have many patients M: One physician can perform many operations and one operation can be performed by many physicians M: One doctor can have specialities in many diseases and one disease can be the speciality of many doctors M: In this section we examine the minimum number of entities in a relationship.
Existence is given as optional, mandatory, or unknown. This is best clarified with an example. Consider again the example discussed in Section 2. Specifically, focus on the manage relationship between department and employee. We know the cardinality is 1: This tells us that at most one department is managed by an employee and an employee can manage, at most, one department. Be sure you understand the distinction between these two phrases. The existence of this relationship tells us the fewest number of departments that can be managed by an employee and the fewest number of employees that can manage a department.
Only one of the following can be true: Similarly, only one of the following may be true: For each set of three above, which ones would you choose? It is not entirely clear from the situation description which of the above are true. I make the relatively standard assumptions that a department must have at least one manager and that an employee need not be the manager of any department. Thus, the existence of this relationship is mandatory in one direction and optional in the other.
Going back to the definition of existence, we can also look at this situation in this way: Given any randomly chosen department, there must be an employee on the other side of the manage relationship. Thus, the relationship is mandatory in this direction. Given any randomly chosen employee, there need not be any department on the other side of the manage relationship. Thus, the relationship is optional in this direction.