23.5. The union Element
A union is a simple type that specifies a series of allowable other simple types as possible values. It is implemented in C and C++ as a union and in Oracle as the SYS.AnyData type. From a database design perspective, unions should be avoided, if possible, due to their design and implementation complexity. This section details a number of possible implementations for two fairly straightforward cases.
Warning
The complex cases of the union element such as a union of lists or a union of IDREFS and NMTOKENS are problematic to properly design and implement in a relational database. Clear representation, easy inserts, updates, and deletes, and proper datatype validation of the union value (which itself is composed of a series ...
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