Information systems have evolved from
simple file processing systems to powerful database systems.
Relational Databases put the layout and form into the
unchanging metadata and gave us "record once" systems.
Unfortunately, the "real world" upon which databases are
modelled constantly changes. These changes may affect the
schema for a variety of reasons. Different formalisms have
been developed to deal with schema changes aiming to
preserve information capacity and semantic correctness.
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