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discuss the referential integrity enforcement steps taken by the DBMS in terms of our Enrolled and Students tables, with the foreign key constraint that Enrolledsid is a reference to (the primary key of) Students In addition to the instance S1 of Students, consider the instance of Enrolled shown in Figure 34 Deletions of Enrolled tuples do not violate referential integrity, but insertions of Enrolled tuples could The following insertion is illegal because there is no student with sid 51111: INSERT INTO Enrolled (cid, grade, sid) VALUES ( Hindi101 , B , 51111) On the other hand, insertions of Students tuples do not violate referential integrity although deletions could Further, updates on either Enrolled or Students that change the sid value could potentially violate referential integrity SQL-92 provides several alternative ways to handle foreign key violations We must consider three basic questions: 1 What should we do if an Enrolled row is inserted, with a sid column value that does not appear in any row of the Students table In this case the INSERT command is simply rejected 2 What should we do if a Students row is deleted The options are: Delete all Enrolled rows that refer to the deleted Students row Disallow the deletion of the Students row if an Enrolled row refers to it Set the sid column to the sid of some (existing) default student, for every Enrolled row that refers to the deleted Students row For every Enrolled row that refers to it, set the sid column to null In our example, this option con icts with the fact that sid is part of the primary key of Enrolled and therefore cannot be set to null Thus, we are limited to the rst three options in our example, although this fourth option (setting the foreign key to null) is available in the general case 3 What should we do if the primary key value of a Students row is updated The options here are similar to the previous case SQL-92 allows us to choose any of the four options on DELETE and UPDATE For example, we can specify that when a Students row is deleted, all Enrolled rows that refer to it are to be deleted as well, but that when the sid column of a Students row is modi ed, this update is to be rejected if an Enrolled row refers to the modi ed Students row:
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CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(10), PRIMARY KEY (sid, cid), FOREIGN KEY (sid) REFERENCES Students ON DELETE CASCADE ON UPDATE NO ACTION )
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The options are speci ed as part of the foreign key declaration The default option is NO ACTION, which means that the action (DELETE or UPDATE) is to be rejected Thus, the ON UPDATE clause in our example could be omitted, with the same e ect The CASCADE keyword says that if a Students row is deleted, all Enrolled rows that refer to it are to be deleted as well If the UPDATE clause speci ed CASCADE, and the sid column of a Students row is updated, this update is also carried out in each Enrolled row that refers to the updated Students row If a Students row is deleted, we can switch the enrollment to a default student by using ON DELETE SET DEFAULT The default student is speci ed as part of the de nition of the sid eld in Enrolled; for example, sid CHAR(20) DEFAULT 53666 Although the speci cation of a default value is appropriate in some situations (eg, a default parts supplier if a particular supplier goes out of business), it is really not appropriate to switch enrollments to a default student The correct solution in this example is to also delete all enrollment tuples for the deleted student (that is, CASCADE), or to reject the update SQL also allows the use of null as the default value by specifying ON DELETE SET NULL
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