Checking Referential Integrity on Database modification The following tests must be made in order to preserve the following referential integrity constraint ∏(r2)≤Ik(r1) Insert. If a tuple t, is inserted into r, the system must ensure that there is a tuple t, in G, such that t,K= t2[a]. That is t2[cx]∈k(r1) Delete. If a tuple, t, is deleted from r1, the system must compute the set of tuples in r2 that reference t, t1] If this set is not empty either the delete command is rejected as an error, or the tuples that reference t, must themselves be deleted (cascading deletions are possible Database System Concepts 6.6 @Silberschatz, Korth and Sudarshan
Database System Concepts 6.6 ©Silberschatz, Korth and Sudarshan Checking Referential Integrity on Database Modification The following tests must be made in order to preserve the following referential integrity constraint: (r2 ) K (r1 ) Insert. If a tuple t2 is inserted into r2 , the system must ensure that there is a tuple t1 in r1 such that t1 [K] = t2 []. That is t2 [] K (r1 ) Delete. If a tuple, t1 is deleted from r1 , the system must compute the set of tuples in r2 that reference t1 : = t1[K] (r2 ) If this set is not empty either the delete command is rejected as an error, or the tuples that reference t1 must themselves be deleted (cascading deletions are possible)
Database Modification(Cont) Update. There are two cases If a tuple t2 is updated in relation r2 and the update modifies values for foreign key a, then a test similar to the insert case is made Let t2 denote the new value of tuple t2. The system must ensure that t2]∈Ik(r1 If a tuple t, is updated in r1, and the update modifies values for the primary key(K), then a test similar to the delete case is made 1. The system must compute a=tiKI (2) using the old value of t,(the value before the update is applied) 2. If this set is not empty the update may be rejected as an error, or the tuples in the set may be deleted Database System Concepts OSilberschatz. Korth and Sudarshan
Database System Concepts 6.7 ©Silberschatz, Korth and Sudarshan Database Modification (Cont.) Update. There are two cases: If a tuple t2 is updated in relation r2 and the update modifies values for foreign key , then a test similar to the insert case is made: Let t2 ’ denote the new value of tuple t2 . The system must ensure that t2 ’[] K(r1 ) If a tuple t1 is updated in r1 , and the update modifies values for the primary key (K), then a test similar to the delete case is made: 1. The system must compute = t1[K] (r2 ) using the old value of t1 (the value before the update is applied). 2. If this set is not empty 1. the update may be rejected as an error, or 2. the tuples in the set may be deleted
Referential Integrity in SQL Primary and candidate keys and foreign keys can be specified as part of the sQL create table statement: The primary key clause lists attributes that comprise the primary key The unique key clause lists attributes that comprise a candidate key The foreign key clause lists the attributes that comprise the foreign key and the name of the relation referenced by the foreign key By default, a foreign key references the primary key attributes of the referenced table foreign key(account-number references account Short form for specifying a single column as foreign key account-number char(10) references account Reference columns in the referenced table can be explicitly specified but must be declared as primary/candidate keys foreign key(account-number references account(account-number Database System Concepts 6.8 OSilberschatz. Korth and Sudarshan
Database System Concepts 6.8 ©Silberschatz, Korth and Sudarshan Referential Integrity in SQL Primary and candidate keys and foreign keys can be specified as part of the SQL create table statement: The primary key clause lists attributes that comprise the primary key. The unique key clause lists attributes that comprise a candidate key. The foreign key clause lists the attributes that comprise the foreign key and the name of the relation referenced by the foreign key. By default, a foreign key references the primary key attributes of the referenced table foreign key (account-number) references account Short form for specifying a single column as foreign key account-number char (10) references account Reference columns in the referenced table can be explicitly specified but must be declared as primary/candidate keys foreign key (account-number) references account(account-number)
Referential Integrity in SQL-Example create table account (account-number char (10) branch-namechar (15) balance Integer primary key(account-number) foreign key(branch-name) references branch create table depositor (customer-name char(20) account-number char(10) primary key(customer-name, account-number foreign key(account-number) references account, foreign key (customer-name)references customer Database System Concepts 6.9 OSilberschatz. Korth and Sudarshan
Database System Concepts 6.9 ©Silberschatz, Korth and Sudarshan Referential Integrity in SQL – Example create table account (account-number char(10), branch-namechar(15), balance integer, primary key (account-number), foreign key (branch-name) references branch) create table depositor (customer-name char(20), account-number char(10), primary key (customer-name, account-number), foreign key (account-number) references account, foreign key (customer-name) references customer)
Cascading Actions in SQL create table account foreign key(branch-name) references branch on delete cascade on update cascade Due to the on delete cascade clauses if a delete of a tuple in branch results in referential-integrity constraint violation. the delete" cascades" to the account relation, deleting the tuple that refers to the branch that was deleted Cascading updates are similar 标 Database System Concepts 6.10 @Silberschatz, Korth and Sudarshan
Database System Concepts 6.10 ©Silberschatz, Korth and Sudarshan Cascading Actions in SQL create table account . . . foreign key(branch-name) references branch on delete cascade on update cascade . . . ) Due to the on delete cascade clauses, if a delete of a tuple in branch results in referential-integrity constraint violation, the delete “cascades” to the account relation, deleting the tuple that refers to the branch that was deleted. Cascading updates are similar