Adding existing relationships to a diagram in PowerDesigner | Metadata Matters, by @MetadataJunkie
of a relational database schema to a graphical entity-relationship diagram. The following adds the name and height attributes to the Person entity: [Person]. Creating an entity-relationship (ER) model is to visually represent the structure of a Each entity must have a name, expressed by a noun in the singular. An entity relationship model, also called an entity-relationship (ER) In our example, the dependent name is unique for every employee.
The Entity-Relationship Model
I was used to a command in Oracle Designer that would add all missing relationships including inheritances to the diagram — every existing relationship that connects two entities that already appear on the diagram would be included. After a search, I found the equivalent command in PowerDesigner.
- Entity Relationship Diagram Symbols
However, I often just wanted to include selected relationships on the diagram, so I tried dragging them onto the diagram from the browser. One has to manually delete the extra ones; a nuisance When you drag a relationship onto a diagram from the browser, PD will automatically include the two entity symbols as well, even if the entities already appear on the diagram.
If you do create unwanted graphical synonyms, all you need to do is delete the new entity symbols, and the relationship symbol will move to the original entity symbols. This could be a real pain in the neck if you added a lot relationships at the same time.
How well do you know your model? In this model, the relationship names are short, and we have some duplicate names as well.
It will also force you to stop giving your relationships the same name, which is no bad thing in itself. Now, you could change the relationship names to include one or both of the entity names which would certainly make the browser list easier to use: I very rarely look at the list of relationships in the browser. 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'.
Entity Relationship Diagram Symbols | Professional ERD Drawing
It occurs with a master table that links to multiple tables in a one-to-many relationship. 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'. A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.
For example, a Building has one-or-more Rooms, that hold zero-or-more Computers. One would expect to be able to query the model to see all the Computers in the Building.
However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list. Another relation between Building and Computers is needed to capture all the computers in the building. This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model. See Entity-Relationship Modelling 2 for details. Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model".
It is an intensional model.
At the latest since Carnapit is well known that: The first part comprises the embedding of a concept in the world of concepts as a whole, i.