Editing Federated database system (section)

==== Schema matching, schema mapping ====
Dealing with incompatible data types or query syntax is not the only obstacle to a concrete implementation of an FDBS. In systems that are not planned top-down, a generic problem lies in matching [[semantic equivalence|semantically equivalent]], but differently named parts from different [[logical schema|schemas]] (=data models) (tables, attributes). A pairwise mapping between ''n'' attributes would result in <math>n (n-1) \over 2</math> mapping rules (given equivalence mappings) - a number that quickly gets too large for practical purposes. A common way out is to provide a global schema that comprises the relevant parts of all member schemas and provide mappings in the form of [[database view]]s. Two principal approaches depend on the direction of the mapping:
# Global as View (GaV): the global schema is defined in terms of the underlying schemas
# Local as View (LaV): the local schemas are defined in terms of the global schema
Both are examples of [[data integration]], called the [[schema matching]] problem.