Entity Relationship Diagram
Purpose
Entity Relationship Diagrams (ERDs) can be considered as graphical representations of DD Entries. Information modelling is concerned with the definition of data within the system in terms of its meaning, composition and relationships. One of the methods within Cradle that can be used to represent information modelling is the use of an ERD. ERDs show graphically how different pieces of information are related.
Example
Here is an example Entity Relationship Diagram.
Description
DFDs show the flow of information through the system but play no part in defining the information, apart from being a useful place from which to access DD Entries. Flows and stores on DFDs are essentially representations of information that is defined elsewhere.
While the DFD models the active processing of information by the system, the ERD models the static relationships amongst this information that are preserved and maintained by the system. The ERD shows how items of data relate, statically, to each other. ERDs cannot exist in a hierarchy, instead, either a single ERD is produced for the entire system analysis or design (when the ERD is considered to relate to the entire DFD hierarchy), and/or ERDs can be produced as companions to specific DFDs that contain a large quantity of stored data, and contain the processes that create, update, or otherwise maintain the stored data.
Characteristics
APPEARS IN MODELS |
Implementation |
NUMBERING |
Cradle does not restrict the numbering of ERDs, so they can be used in whatever manner is appropriate. |
HIERARCHICAL |
No |
 |
Comment |
Makes a note anywhere in the diagram. Are always surrounded by * characters. |
None |
None |
 |
Data object |
An item of data operated upon by the system. |
DD Entry |
None |
 |
Relationship |
A relationship between one or more Data Objects , as one of: one-to-one, one-to-many, many-to-many. |
DD Entry |
None |
 |
Subtype/Supertype relationship |
A relationship between (normally) three or more Data Objects , used in the situation where one Data Object contains data elements common to two or more other Data Objects (such as an object machine containing data items price , weight , and size , and two other Data Objects , lathe and drill , containing data elements unique to each machine type). |
None |
None |
 |
Picture |
Allows you to choose the location of a GIF or JPEG image to be displayed as a diagram symbol or to be embedded in an existing diagram symbol. |
None |
None |
 |
Connection symbol |
A link between a Data Object and a Relationship . |
None |
None |
 |
Subtype/Supertype connection |
A link between Data Objects and Relationships or Sub/Supertype Relationships . |
None |
None |
 |
Associative connection |
A link between an unnamed Relationshop and a named Data Object that indicates that the Relationship contains data in addition to the relationship between the Data Objects . |
None |
None |
|
