April 2015 Archive

Keys Declared With UNIQUE

One more way to declare a key is to use the keyword UNIQUE. This word can appear exactly where PRIMARY KEY can appear: either following an attribute and its type or as a separate item within a CREATE TABLE statement. The meaning of a UNIQUE declaration is nearly the

Declaring Foreign-Key Constraints

A second important kind of constraint on a database schema is that values for certain attributes must make sense. That is, an attribute like presC# of relation Studio is expected to refer to a particular movie executive. The implied "referential integrity" constraint is that if a studio's

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