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Unformatted text preview: 1 Constraints Superkey a collection of attributes in a relation that uniquely identifes each tuple in it . Candidate key a superkey that has no superkey subsets Primary key one candidate key per relation, designated to be the main way of maintaining tuple uniqueness. Key constraint : each relation must have a primary key . Foreign key a primary key of one relation, included in the attributes of another relation (usually for the purpose of linking two components of the database together). Referential integrity constraint each collection of values of a foreign key in a relation must appear as a primary key in the referenced relation. Null value : a no value value for a relational attribute. Lack of value, or value not yet available. not null constraint : a statement that a speciFc attribute is not allowed to have null values . (e.g., primary key attributes)....
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