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A "relationship" is a "link" between entities in the broadest sense of the word. There are many different types of relationships and what happens with these relationships when you generate a Physical Data Model depends on the properties of each type.
See also: Relationship Types
The Toolbox Toolbar has two buttons to create a Relationship in the Editor Canvas, one for Identifying Relationship and one for a Non-Identifying Relationship. You can switch from an Identifying to a Non-Identifying and vice-versa in the Relationship Properties dialog.
For information on how to work with links in the Diagram Editor, see Creating a Link.
After creating a new relationship, you can double click it to show the Relationship Properties dialog.
Relationship Properties dialog
As all objects, a relationship also has a Name and an area where you can enter a description. On the first page, you will see the checkbox to switch the relationship between Non-Identifying and Identifying.
The second page of the dialog is more interesting. The Cardinalities define the number of instances (zero, one or many) of one entity to another.
Relationship Properties dialog, page Cardinalities
The preview area shows what the relationship will look like in the diagram after you click the OK button. Here you can also check the relationship direction to see if it's correct.
You can use the radio buttons to select your relationship type, this will also change the selected item in the Cardinality comboboxes, as these reflect the relationship type as well. Depending on the type, the Dominant Role box becomes available which tells you what direction the relationship goes.
Captions & additional properties
Using the required check boxes will also modify the Cardinality values and vice versa, you can use whatever suits you.
The Role name boxes allow you to put captions and a "meaning" near the end-points of the relationship.
With the Attributes selection boxes, you can set zero or more attributes to become the attributes used in this relationship. If you do not enter any attributes here, but leave it set to "default", the attributes will be generated when creating a physical model.