Chapter 3 Building an XML model
A key constraint specifies that an element or an attribute value (or set of values) must be a key within a specified scope.
A key means that data should be unique, not null and always present within a specified scope.
For example:
Generated schema:
The KEYCODE key constraint, defined on the project element, specifies that the code attribute must be unique, not null and always present within the product element.
To display a key property sheet, double-click its name or its icon in the Browser tree view.
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