It is a failure to place information about a new database entryinto all the places in the database where information about the new entry needs to be stored. In a properly normalized database, information about a new entry needs to be inserted into only one place in the database, in an inadequatly normalized database, information about a new entry may need to be inserted into more than one place, and human fallibility being what it is, some of the needed additional insertionsmay be missed.
It is a failure to remove information about an existing database entry when it is time to remove that entry. In a properly normalized database, information about an old, to-be-gotten-rid-of entry needs to be deletedfrom only one place in the database, in an inadequatly normalized database, information about that old entry may need to be deleted from more than one place.
An update of a database involves modifications that may be additions, deletions, or both. Thus “update anomalies” can be either of the kinds discussed above.
All three kinds of anomalies are highly undesirable, since thier occurence constitutes corruption of the database. Properly normalized database are much less susceptible to corruption than are un-normalized databases.