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If a client application updates or deletes a primary key value in a table, and if a foreign key references that primary key value elsewhere in the database, there is a danger of a breach of referential integrity.
If the server allowed the primary key to be updated or dropped, and made no alteration to the foreign keys that referenced it, the foreign key reference would be invalid. Any attempt to use the foreign key reference, for example in a SELECT statement using a KEY JOIN clause, would fail, as no corresponding value in the referenced table exists.
While SQL Anywhere handles breaches of entity integrity in a generally straightforward fashion by simply refusing to enter the data and returning an error message, potential breaches of referential integrity become more complicated. You have several options (known as referential integrity actions) available to help you maintain referential integrity.