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©Silberschatz, Korth and Sudarshan6.1Database System Concepts Chapter 6: Integrity (and Security) Domain Constraints Referential Integrity Assertions Triggers
©Silberschatz, Korth and Sudarshan6.2Database System Concepts Domain Constraints Integrity constraints guard against accidental damage to the database, by ensuring that authorized changes to the database do not result in a loss of data consistency. Domain constraints are the most elementary form of integrity constraint. They test values inserted in the database, and test queries to ensure that the comparisons make sense. New domains can be created from existing data types E.g. create domain Dollars numeric(12, 2) create domain Pounds numeric(12,2) We cannot assign or compare a value of type Dollars to a value of type Pounds. However, we can convert type as below (cast r.A as Pounds) (Should also multiply by the dollar-to-pound conversion-rate)
©Silberschatz, Korth and Sudarshan6.3Database System Concepts Domain Constraints (Cont.) The check clause in SQL-92 permits domains to be restricted: Use check clause to ensure that an hourly-wage domain allows only values greater than a specified value. create domain hourly-wage numeric(5,2) constraint value-test check(value > = 4.00) The domain has a constraint that ensures that the hourly-wage is greater than 4.00 The clause constraint value-test is optional; useful to indicate which constraint an update violated. Can have complex conditions in domain check create domain AccountType char(10) constraint account-type-test check (value in (‘Checking’, ‘Saving’)) check (branch-name in (select branch-name from branch))
©Silberschatz, Korth and Sudarshan6.4Database System Concepts Referential Integrity Ensures that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation. Example: For each DeptNo in the EMP relation there must exists a tuple in the DEP table describing that department existence dependency. DeptNo should also be the key for DEP. Formal Definition Let r 1 (R 1 ) and r 2 (R 2 ) be relations with primary keys K 1 and K 2 respectively. The subset of R 2 is a foreign key referencing K 1 in relation r 1, if for every t 2 in r 2 there must be a tuple t 1 in r 1 such that t 1 [K 1 ] = t 2 [ ]. Referential integrity constraint also called subset dependency since its can be written as (r 2 ) K1 (r 1 ) Referential integrity= Foreign Key constraint.
©Silberschatz, Korth and Sudarshan6.5Database System Concepts Enforcing FK on Database Modification Insert. If a tuple is inserted in DEP no problem. If it is inserted in EMP, then its DepNo value should be in DEP—otherwise FK violation and error. Delete. If a tuple is deleted from EMP no problem. If it is deleted from DEP: FK violation and error unless we: 1. Change the DepNo of matching records in EMP value to NULL, or 2. Delete the matching records in EMP! Update: same as delete followed by insert.
©Silberschatz, Korth and Sudarshan6.6Database System Concepts FK in SQL Primary and candidate keys and foreign keys can be specified as part of the SQL create table statement: The primary key clause of the create table statement includes a list of the attributes that comprise the primary key. The unique key clause of the create table statement includes a list of the attributes that comprise a candidate key. The foreign key clause of the create table statement includes both a list of the attributes that comprise the foreign key and the name of the relation referenced by the foreign key.
©Silberschatz, Korth and Sudarshan6.7Database System Concepts Referential Integrity (FK) in SQL – e.g. create table customer (customer-namechar(20), customer-streetchar(30), customer-citychar(30), primary key (customer-name)) create table branch (branch-namechar(15), branch-citychar(30), assetsinteger, primary key (branch-name))
©Silberschatz, Korth and Sudarshan6.8Database System Concepts Referential Integrity in SQL – Example (Cont.) create table account (account-numberchar(10), branch-namechar(15), balanceinteger, primary key (account-number), foreign key (branch-name) references branch) create table depositor (customer-namechar(20), account-numberchar(10), primary key (customer-name, account-number), foreign key (account-number) references account, foreign key (customer-name) references customer)
©Silberschatz, Korth and Sudarshan6.9Database System Concepts 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.
©Silberschatz, Korth and Sudarshan6.10Database System Concepts Cascading Actions in SQL (Cont.) If there is a chain of foreign-key dependencies across multiple relations, with on delete cascade specified for each dependency, a deletion or update at one end of the chain can propagate across the entire chain. If a cascading update to delete causes a constraint violation that cannot be handled by a further cascading operation, the system aborts the transaction. As a result, all the changes caused by the transaction and its cascading actions are undone. Referential integrity should only be checked at the end of a transaction (not all DSMS follow this policy) Intermediate steps are allowed to violate referential integrity provided later steps remove the violation Otherwise it would be impossible to create some database states, e.g. insert two tuples whose foreign keys point to each other (e.g. spouse attribute of relation marriedperson)
©Silberschatz, Korth and Sudarshan6.11Database System Concepts Referential Integrity in SQL (Cont.) Alternative to cascading: on delete set null on delete set default Null values in foreign key attributes complicate SQL referential integrity semantics, and are best prevented using not null if any attribute of a foreign key is null, the tuple is defined to satisfy the foreign key constraint!
©Silberschatz, Korth and Sudarshan6.12Database System ConceptsAssertions An assertion is a predicate expressing a condition that we wish the database always to satisfy. An assertion in SQL takes the form create assertion check When an assertion is made, the system tests it for validity, and tests it again on every update that may violate the assertion This testing may introduce a significant amount of overhead; hence assertions should be used with great care. Asserting for all X, P(X) is achieved in a round-about fashion using not exists X such that not P(X)
©Silberschatz, Korth and Sudarshan6.13Database System Concepts Assertion Example The sum of all loan amounts for each branch must be less than the sum of all account balances at the branch. create assertion sum-constraint check (not exists (select * from branch where (select sum(amount) from loan where loan.branch-name = branch.branch-name) >= (select sum(amount) from account where account.branch-name = branch.branch-name)))
©Silberschatz, Korth and Sudarshan6.14Database System Concepts Assertion Example Every loan has at least one borrower who maintains an account with a minimum balance or $ create assertion balance-constraint check (not exists ( select * from loan where not exists ( select * from borrower, depositor, account where loan loan-number = borrower loan-number and borrower customer-name = depositor customer-name and depositor account-number = account.account-number and account balance >= 1000)))
©Silberschatz, Korth and Sudarshan6.15Database System ConceptsTriggers A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database. To design a trigger mechanism, we must: Specify the conditions under which the trigger is to be executed. Specify the actions to be taken when the trigger executes. Triggers introduced to SQL standard in SQL:1999, but supported even earlier using non-standard syntax by most databases.
©Silberschatz, Korth and Sudarshan6.16Database System Concepts Trigger Example Suppose that instead of allowing negative account balances, the bank deals with overdrafts by setting the account balance to zero creating a loan in the amount of the overdraft giving this loan a loan number identical to the account number of the overdrawn account The condition for executing the trigger is an update to the account relation that results in a negative balance value.
©Silberschatz, Korth and Sudarshan6.17Database System Concepts External World Actions We sometimes require external world actions, such as re-ordering an item whose quantity in a warehouse has become small, or turning on an alarm light, to be triggered on a database update Triggers cannot be used to directly implement external-world actions, BUT Triggers can be used to record actions-to-be-taken in a separate table, and we can have an external process that repeatedly scans the table and carries out external-world actions. E.g. Suppose a warehouse has the following tables inventory(item, level): How much of each item is in the warehouse minlevel(item, level) : What is the minimum desired level of each item reorder(item, amount): What quantity should we re-order at a time orders(item, amount) : Orders to be placed (read by external process)
©Silberschatz, Korth and Sudarshan6.18Database System Concepts When Not To Use Triggers Triggers were once used for tasks such as maintaining summary data (e.g. total salary of each department) Replicating databases by recording changes to special relations (called change or delta relations) and having a separate process that applies the changes over to a replica There are better ways of doing these now: Databases today provide built in materialized view facilities to maintain summary data Databases provide built-in support for replication Encapsulation facilities can be used instead of triggers in many cases Define methods to update fields Carry out actions as part of the update methods instead of through a trigger
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