SDB_Key - Key definition

A key is a projection from the associated complex data type, i.e. it consists of a subset of properties (key components) of the complex data type. Key components usually refer to attributes. Only one key component may refer to a generic attribute or to a reference. Keys may also refer to transient attributes. Keys may not refer to relationships.

Keys form complex data types. Key values are provided as (key) instances.

When referring to attribute arrays, references or generic attributes, a multiple key is created, which creates as many key instances as there are collection elements or generic attribute instances for each instance.

Specific keys are __IDENTITY and __AUTOIDENT keys. __IDENTITY keys are keys that refer to the identity as key attribute. Identity keys do not have components. __AUTOIDENT keys are keys, which have got the __AUTOIDENT attribute as first key component. __AUTOIDENT keys provide automatic numbers for instances created in a collection.

Key components may define ascending (default) or descending order. Since this can be defined on the level of key components, it is not the key itself, which is ascending or descending. Those modes can also be mixed, which creates ascending/descending keys. Since this is not an index feature, several keys with the same attributes must be defined for supporting ascending and descending orders at the same time.