Chapter 6: Integrity Objective Key Constraints (Chapter 2) Cardinality Constraints (Chapter 2) Domain Constraints Referential Integrity Assertions Triggers

Integrity Constraints Objective –Maintain DB consistency –Ensure that authorized changes to the database do not result in a loss of data consistency

Domain Constraints Domain constraints –The most elementary form of integrity constraint –Test values & queries Domain Constraints –New domains can be created from existing data types –E.g. create domain Dollars numeric(12, 2) create domain Pounds numeric(12,2) Type conversion

Domain Constraints (Cont.) The check clause in SQL-92 –create domain hourly-wage numeric(5,2) constraint value-test check(value > = 4.00) –constraint: the hourly-wage >= 4.00 –constraint value-test is optional –A domain check can have complex conditions create domain AccountType char(10) constraint account-type-test check (value in (‘Checking’, ‘Saving’)) check (branch-name in (select branch-name from branch))

Referential Integrity Foreign key constraint –Assume a branch name “Perryridge” appears in the account relation –“Perryridge” should also appear in branch relation Formal definition –Foreign key The subset of R2’s attributes The primary key of R1

Referential Integrity and the E-R Model Consider relationship set R between entity sets E 1 and E 2 –The schema of R should include the primary keys K 1 of E 1 and K 2 of E 2 –K 1 and K 2 form foreign keys on the relational schemas for E 1 and E 2 respectively R E1E2

Referential Integrity and the E-R Model (Cont.) Weak entity sets –The schema of a weak entity set must include the primary key of the entity set on which it depends –Descriminator

Checking Referential Integrity on DB Modification Rule of Thumb –Maintain the subset dependency   (r 2 )   K (r 1 ) Insert –When inserting a tuple t 2 into r 2, ensure that there is a tuple t 1 in r 1 such that t 1 [K] = t 2 [  ]: t 2 [  ]   K (r 1 )

DB Modification (Cont.) Delete –If a tuple t 1 is deleted from r 1, the system must compute the set of tuples in r 2 that reference t 1 :   = t1[K] (r 2 ) If this set is not empty –reject the delete command as an error, or –delete the tuples that reference t 1 (cascading deletions)

Database Modification (Cont.) Update –Case 1: If the value of the foreign key  is modified, ensure that t 2 ’[  ]   K (r 1 )

Database Modification (Cont.) Update –Case 2: If the update modifies the value of a primary key, compute   = t1[K] (r 2 ) using the old value of t 1 [K] If this set is not empty –the update may be rejected as an error, –the update may be cascaded to the tuples in the set, or –the tuples in the set may be deleted.

Referential Integrity in SQL Specify keys in the create table statement: –primary key clause –unique key clause –foreign key clause foreign key (account-number) references account(account-number) foreign key (account-number) references account

Referential Integrity in SQL 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))

Referential Integrity in SQL 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)

Cascading Actions in SQL create table account... foreign key(branch-name) references branch on delete cascade on update cascade... ) If a delete of a tuple in branch results in referential-integrity constraint violation, the delete “cascades” to the account relation A chain of foreign dependency

Cascading Actions in SQL (Cont.) Referential integrity –Check at the end of a transaction –Intermediate steps may violate referential integrity if later steps remove the violation –E.g. insert two tuples whose foreign keys point to each other spouse attribute of relation marriedperson(name, address, spouse) Any other example? Is this a good idea?

Alternatives to Cascading on delete set null, e.g., null branch-name on delete set default Null values in foreign key attributes complicate SQL referential integrity semantics –Use not null to prevent

Assertions A predicate expressing a condition that DB always need to satisfy create assertion check System tests the validity –May introduce a significant overhead

Assertion Example Sum(all loan amounts for each branch) < Sum(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 loan.branch-name = branch.branch-name)))

Triggers An automatically executed statementment as a side effect of a modification to the database –Specify the conditions under which the trigger is to be executed –Specify the actions Example –Conidtion: an overdraft –Possible actions: Set the account balance to zero Create a loan in the amount of the overdraft

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