CP363 Lecture Notes - Lecture 8: Referential Integrity, Data Integrity, Unique Key
8 Apr 2019
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The primary key attributes pk of each relation schema r in s cannot have null values in any tuple or r(r) This is because primary key values in any tuple of r(r) This is because primary key values are used to identify the individual tuples t[pk} is never equal to null for any tuple t in r(r) If pk has several attributes null is not allowed in any of these attributes. Note: other attributes of r may be constrained to disallow null values. Referential integrity even though they are not members of the primary key. The previous constraints involve a single relation. Used to specify a relationship among tuples in two relations. The referencing relation and the referenced relation. Tuples in the referencing relation r1 have attributes fk (called foreign key attributes) that reference the primary key attributes pk of the referenced relation r2.
