A foreign key consists of a column or set of columns. It represents a reference to a row in the primary table with the matching key value. Foreign keys can only be used with base tables; they cannot be used with temporary tables, global temporary tables, views, or materialized views. A foreign key is sometimes called a referential constraint as the base table containing the foreign key is called the referencing table and the table containing the primary key is called the referenced table.
If the foreign key is nullable, then the relationship is optional as the foreign row may exist without a corresponding match of a primary key value in the referenced table since neither primary keys nor UNIQUE constraints columns can be NULL. If foreign key columns are declared NOT NULL, then the relationship is mandatory and each row in the referencing table must contain a foreign key value that exists as a primary key in the referenced table.
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