Rules for Referential Integrity Constraints
Referential integrity constraints must obey the following set of rules:
The parent key must exist when the referential integrity constraint is defined.
The parent key columns must be either a unique primary index (UPI) or a unique secondary
The foreign and parent keys must have the same number of columns and their data types
The foreign and parent keys cannot exceed 64 columns.
Duplicate referential integrity constraints are not allowed.
The columns on which a referential integrity relationship is defined cannot be defined
with the XML, BLOB, CLOB, ARRAY, VARRAY, or Geospatial data types.
You cannot drop or alter foreign key or parent key columns using an ALTER TABLE statement
after a referential integrity constraint has been defined on them.
To drop a foreign or parent key column after a referential integrity constraint has
been defined on it, you must first drop the referential constraint and then alter
the table to drop the foreign or parent key columns.
A foreign key value must be equal to its parent key value or it must be null.
Self-reference (a condition in which the Parent and Child tables are the same table)
is allowed, but the foreign and parent keys cannot consist of identical columns.
You can define no more than 64 referential integrity constraints per table.
You cannot define a referential integrity relationship on a global temporary trace
table. See “CREATE GLOBAL TEMPORARY TRACE TABLE” in SQL Data Definition Language Detailed Topics.