The relational model

Question	Answer
What is a relation?	Defined as a subset of the product set
What is a a tuple?	Members of a relation
Describe 1NF shortly.	Only simple/atomic values are allowed in the relational model.
When do you use the special value NULL?	An unknown value, a missing value or an attribute that is not applicable
Explain super key	A set of attributes that allows us to do it. A relation can have many superkeys.
Explain Candidate key	A minimal superkey. A relation can have many candidate keys, normally, less then the superkeys.
Explain Primary key	The single candidate key we choose to identify records in a table, no NULLs allowed.
Explain delete reactions: Cascade Set NULL Set default No action	cascade: deletes are propagated. set null: the referring attribute values are set to null. set default: the referring attribute values are set to their default. no action: deletion is not authorized.
Explain update reactions: cascade set null set default no action	cascade: the value is updated also on the referring attributes. set null: the referring attribute values are set to null. set default: the referring attribute values are set to their default. no action: deletion is not authorized.
Explain foreign key	used to “connect” relations and reference tuples in other relations.
Explain partial key	Specifies a key that that is only partially unique. Used for weak entities. It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity.

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	Created by linneamd about 8 years ago