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PowerDesigner 7.x Conceptual Data Model - Frequently Asked Questions

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General Topics


Q.  Why does PowerDesigner have a two-level design?

A.  PowerDesigner is based on the Information Engineering Methodology by James Martin. This methodology is based on a two-level design consisting of a Conceptual Data Model (CDM) and a Physical Data Model (PDM). The aim in separating the database design into a two-step procedure is to greatly simplify the design process that would otherwise be more complicated. The CDM allows the designer to concentrate solely on defining the objects in the information system and the relationships between them, without having to consider the numerous parameters associated with the physical implementation such as data integrity constraints, data access speed and data storage efficiency. The CDM thus provides a clear and succinct picture of the information system, which is independent of the targeted DBMS. A single CDM may therefore be associated with a number of PDMs targeting different DBMS.

Q. What is an Inheritance Relationship (or Super-type / Sub-type relationship)?

A. An inheritance relationship describes the relationship between a super-type entity and its sub-type entities. As an example, take the case of the book entity. We might define two sub-type entities, play and novel, for this entity, which then becomes a super-type entity.

The idea is that the super-type entity will contain the principal characteristics (attributes) defining the modeled object, whereas each sub-type entity will describe the characteristics specific to its particular sub-group. In our example, the book entity might contain the name, number of pages and author of the book. The play sub-type entity might contain an attribute describing the number of acts in the play and the novel sub-entity might contain an attribute describing the number of chapters in the novel.

In the inheritance definition you may indicate how the inheritance relationship will be implemented physically. You may choose to generate the parent table, the child tables or both and the PDM will reflect this choice.
 

Q. What is a Dependent Relationship in PowerDesigner?

A. A Dependent Relationship from entity A to entity B defines A as partly or wholly identified by B's identifier. As an example, an order line might be identified by its order line number and the order number. This would be represented by a dependent relationship between the order line entity and the order entity.

This CDM will produce the following PDM:

Note that the Order primary key migrates to the Order Line table to become part of its primary key.
 
 

Q. Does PowerDesigner have Associative Entities?

A. Yes, you can transform a relationship into an associative entity linked by two relationships. The associative entity gets the name and code of the relationship.

To change a relationship into an associative entity:

1) Right-click a relationship line.
The relationship context menu appears

2) Select Change to Entity from the context menu.
An associative entity with two relationships replaces the relationship. The associative entity takes the name of the original relationship.


 

Q. Does PowerDesigner allow attributes on relationships?

A. No, although this addition is planned for a future release of PowerDesigner. However, it is possible to use an ordinary entity to resolve this situation. Simply create an entity with dependent relationships to the entities participating in the relationship to be given an attribute, as shown below:


 
 

Q. What is the difference between a Primary Key and an Alternate Key?

A. Any column or set of columns, whose combined values uniquely identify a table row, is termed a Candidate Key. One of these keys is chosen to become the primary key and the others are then alternate keys. These alternate keys may be used, as well as the primary key, to locate a table row. For example, the Client table might have Client Number as its primary key and Client Name as an alternate key. Both keys will uniquely identify a client in the client table.
 
 

Q.  Does PowerDesigner support Object Oriented modeling?

A. In a Conceptual Data Model, PowerDesigner already supports a number of Object Oriented concepts such as inheritance relationships and dependent relationships (a restricted form of inheritance - only identifiers inherited). PowerDesigner 7.x also has support for an Object Oriented Model (OOM) which currently supports the modeling of UML Class diagrams.  Depending on the PowerDesigner product you purchased, you may or may not have this functionality.
 
 

Q.  Why doesn't PowerDesigner support SSADM / ORM / E-R?

A. PowerDesigner supports the Information Engineering Methodology by James Martin. The choice of methodology was based on the popularity of this method throughout the world and its capacity to produce a clear and succinct design even for complicated systems. All the important elements in Entity-Relation modeling are present in this methodology and a number of Object Oriented concepts such as inheritance.
 

Q.  Where are the foreign keys in the CDM?

A.  Foreign keys are found in the PDM and not the CDM. The CDM deals uniquely with entities and relationships and not physical implementation considerations such as foreign keys.  If you want to see foreign keys included in an E/R diagram:

This is the only way to display foreign keys in an E / R format.
 
 

Q.  How do I use Business Rules?

A.  Business rules can be used to describe in more detail the business system that you are modeling. PowerDesigner allows you to define 4 types of business rules - Definition, Fact, Formula and Validation.

A definition type business rule adds to the definition of an object in the information system (e.g. a client is a person who buys one or more products).

A fact type business rule describes a fact pertaining to the information system (e.g. a client may place one or more orders).

A formula type business rule describes a calculation employed in the information system (e.g. the order total is the sum of all the order line costs).

A validation type business rule describes a validity constraint in the information system (e.g. the sum of the order totals for a given client must not be greater than that client's credit allowance).

At the conceptual level, business rules are used to document the behavior of your model. At the Physical Level, you can associate SQL scripts to the rule to actually execute their behavior. These are entered in the Server Expression field and can be implemented in your database either through triggers and procedures as macros or as column constraints by associating validation type rules to columns. Many rules, definitions and facts in particular, might not be implemented in the database structure, but serve to make the database reports more organized and complete.
 

Generation


Q.  Is it possible to generate for different DBMS 's from a given model?

A.  Yes, from a given CDM, you may choose the Generate Physical Data Model command in the Tools menu . Thenselect the target DBMS for which you wish to generate from the proposed list as many times as you want and the Physical models will reflect the chosen target databases.

Once you are in a specified PDM, you would need to select Change Current DBMS from the Database menu and regenerate.  This is not a recommended method for generating to multiple DBMS's since certain properties of a Physical Data Model are altered when you change the DBMS.  This means that depending on the selected DBMS's you may not be able to revert completely to the original. The recommended method would be to maintain a separate PDM for each different target DBMS.
 

Q.  Is it possible to generate for a distributed database?

A.  Yes, This should be done by creating different Physical Data Models for each of the required databases and creating references between objects via shortcuts.  PowerDesigner does not know how to generate references that span databases, so the user will need to manually implement these types of links if they are supported by the target DBMS's.
 

Q.  Can changes made to the PDM go back to the CDM easily (« Synching » between levels)?

A.  It is possible to produce a CDM to conform to changes made in the PDM, by choosing the Generate Conceptual Data Model option in the PDM Tools menu. In the CDM Generation Options dialog box, you can select whether to create a new Conceptual model or update an existing Conceptual Model.  The Update option will present you with the Model Merge screen, which highlights the differences between the existing model and the newly generated model and you can select which changes to incorporate into the Conceptual model.  For more information on Model Merge, see the Comparing and Merging Models chapter of the General Features Guide.
 

Q.  Why is it that sometimes not all the modeling objects are taken into account when I generate from the PDM to the CDM?

A. When generating a CDM from a PDM it is normal to lose some object: The process analyzes tables and references and tries to determine the type of entities and relationships to create in an E-R diagram. This means that associative tables will be replaced by a many-to-many relationship, thus there will not be an entity for that table. Another example of this is that the Conceptual Model does not have foreign keys, but the reference/foreign key combination is replaced with a relationship, thus foreign key columns will not appear in your Conceptual Model.
 

Graphics


Q.  Is there a default graphical layout capability?

A. PowerDesigner 7.x provides a graphical layout capability in a function called Auto-Layout.  A fairly simple algorithm is used which tries to eliminate overlapping objects and crossed lines.
You can invoke the graphical layout capability by performing the following:
1) Select all objects you would like to have arranged.  For all objects select Edit > Select All from the menu.

2) Select Symbol > Auto-Layout from the menu.

Repeated use of this function will provide some number of variants of a layout.  So if you do not like what you see try again.  Remember that you can always Edit > Undo to previous versions.  If none of the variants are to your liking, you can try some manual adjustments to your model then try again, since those changes may alter how the algorithm gets rendered.
 

Q.  What are Synonyms?

A. Synonyms are graphical duplicates of a modeling object (entity/table). They share a single definition in the PowerDesigner dictionary. Any changes made to one synonym will be made to all others. They are used to simplify the schema layout by avoiding long links.  Synonyms can be identified by the (: number) following the Entity/Table name as displayed in the following graphic.


 
 

Q.  How do I change a relationship/reference from one entity/table to another entity/table?

A.  Click on the relationship/reference and while pressing <Ctrl> on the keyboard, select and drag the endpoint of the relationship/reference from the original entity/table to the new entity/table.
 

Diagrams and Packages


Q.  How do I sub-divide my large model into more manageable sections?

A.  PowerDesigner has two concepts to allow sub-division of a model, Diagrams and Packages.  Which one to use depends on your particular requirements.

Packages - A package is a subdivision of a model. When you are working with large models, it is useful to split them into smaller subdivisions in order to avoid manipulating large sets of items. Packages are used to assign different tasks or subject areas, to different development teams.
A model can contain any number of packages. You can build a hierarchy of packages, in which there is no limit to the level of decomposition.
Advantages
Objects are owned by the package.
Report generation, Database generations and metaworks extraction can be selected by package.
Repository access rights can be assigned by package.
Disadvantages
Using objects from other packages requires the use of shortcuts, which can be confusing.

Diagrams - A diagram is a graphical view of a model or package which displays object symbols. Diagrams allows you to split the display of large models and packages in order to focus on certain objects or subject areas. They can also be used to view the symbols of the same objects, displayed with different kinds of information.
Advantages
Shortcuts are not necessary since all objects are in the same area.
Disadvantages
You cannot select for Report generation, Database generations and metaworks extraction based on Diagrams.
You cannot assign Repository access rights based on Diagrams.
 

Q.  How do I reference objects in other packages or models?

A. Shortcuts - A  Shortcut is an object that represents and references a  target object in the same model or in a different model or package. When you modify the definition of a shared object, all its occurrences called shortcuts, are updated in the different models.  Using shortcuts means you do not have to duplicate objects and ensures object synchronization.  The most straight forward method of creating a shortcut is to copy an object from the browser or diagram and Paste as shortcut in the new model section of the browser.  Another method is using drag and drop from one diagram to another.  Drag and drop has a number of different behaviors based on the general options you have set.  Thus if you Drag and do not see the shortcut symbol under the drag icon, either a move or duplicate will occur.  You can force a shortcut by pressing Shift-CTRL while dragging.  For more information see the Managing Shortcuts chapter of the General Features Guide.
 

Q.  If I work in a package, how do I get my work into a global diagram?

A. This is a manual process of copying shortcuts from the individual packages to a diagram at the root level.
 

Q.  Can I have a package within a package?

A. Yes, you can create as many levels of packages as desired.  To do this Right-Click on the desired package, from the menu select New > Package.  It will request a name for the new package, and will always be created with a default diagram or you can place a Package symbol in a diagram and it will create a new package at that level.
 

Q.  Is it possible to have different character setups (fonts etc.) between diagrams or between different models?

A.  Yes, display preferences are stored for each diagram and model. For each diagram, select Tools > Display Preferences and set your desired selections.  Additionally certain characteristics can be set for individual graphic objects.  Either Right-Click on the object and select Format from the context menu or select the object and select Symbol > Format from the menu.
 

Q. Can I extract a package separately from the MetaWorks Repository?

A. Yes, you can select individual packages to extract from MetaWorks.  Note:  If the package you select has shortcuts to other packages within the model, those referenced packages will also be extracted.
 

Q. Can I print a report for only the objects in a package?

A. Yes, Select Report > Select Objects from the Menu.  The Report menu entry is only visible when the Report Editor is active.  Package selection can be made in the dropdown in the toolbar line on the Report Object Selection dialog.
 

Q. Can I print a report or graphics for only the objects in a diagram?

A. Yes for graphics, By simply selecting File > Print from the menu while a diagram is active, you can print the diagram graphics. Not really for reports,  the only way to select objects from a diagram to report is to Edit > Select All in your diagram,  then go to the Report Editor and select Report > Select Objects, then in the Report Object Selection dialog click the Use Graphical Selection button for each tab with graphical objects.  This will select the appropriate graphical object, but will not attempt to filter for non-graphical objects used by the selected graphical objects, i.e. domains, business rules, etc.
 

Q. Can I collapse a package down so that it is just displayed as a single object in a global model?

A. PowerDesigner will not automatically do this, but each package has its own   graphical object which can be placed on a diagram.  You can either use the package tool from the pallete which will create a new package, or if you have already created the package, go to the diagram and select Symbol > Show Symbols and select the package to display on the package tab.
 
 
 
 



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