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Tham khảo tài liệu 'introduction to database systems- p7', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Part II The Relational Model foreign key referring to R n-2 . and so on and finally R1 includes a foreign key referring back to Rn again. More succinctly a referential cycle exists if there is a referential path from some relvar Rn to itself . 9. Foreign-to-candidate-key matches are sometimes said to be the glue that holds the database together. Another way of saying the same thing is that such matches represent certain relationships. Note carefully however that not all such relationships are represented by keys in this way. For example there is a relationship colocation between suppliers and parts represented by the CITY attributes of relvars S and P a given supplier and a given part are colocated if they are located in the same city. However this relationship is not represented by keys. 10. Historically the foreign key concept has been defined for base relvars only a fact that raises some questions in itself see the discussion of The Interchangeability Principle in Chapter 10 Section 10.2 . We do not impose such a restriction here however we ----do limit our discussions to base relvars only where it makes any difference for rea- sons of simplicity. 11. The relational model originally required that foreign keys reference very specifically i - primary keys not just candidate keys see e.g. reference 9.9 once again . We reject that limitation as unnecessary and undesirable in general though it might often constitute good discipline in practice 9.14 . v 12. Along with the foreign key concept the relational model includes the following rule the referential integrity rule . Referential integrity The database must not contain any unmatched foreign key values.17 The term unmatched foreign key value here means a foreign key value in some referencing relvar for which there does not exist a matching value of the relevant candidate key in the relevant referenced relvar. In other words the constraint says simply If B references A then A must exist Here then is the syntax for .
