Integrity Constraints B.Ramamurthy 11/8/2018 B.Ramamurthy

Introduction Integrity constraints make sure that changes to the database do not result in data inconsistency. Key declarations (candidate, primary, foreign) are a basic form of integrity constraint. A Integrity constraint can be any arbitrary predicate pertaining to the database. 11/8/2018 B.Ramamurthy

Domain Constraint Domain constraints allows us to test the values inserted into the database and to test the queries to make sure comparisons made are appropriate. Examples: create domain wage numeric (5,2) constraint wage-value-test check (value >= 4.0) 11/8/2018 B.Ramamurthy

Referential Integrity Let r1(R1) and r2(R2) be relations with primary key K1 and K2 A subset “a” of attributes R2 is a foreign key in relation r1, if for every tuple t2 in r2, there must be a tuple t1 in r1 such that t1[K1] = t2[a] Example: r1 Branch-scheme (branch_name, … r2 Account-schema (account#, branch_name Foreign key 11/8/2018 B.Ramamurthy

Foreign Key  (r2)  K1 (r1) Formally expressing the integrity constraint. For referential constraint to make sense  must be equal to K1. Intuitively, consider a query “insert account xyz with abc as branch” . What if “abc” does not exist? 11/8/2018 B.Ramamurthy

Referential Integrity in SQL When creating table we specify primary , candidate and foreign keys. These are specifying referential integrity. Example Fig6.2 Create table depositor (cname char(20) not null, acctNo char(10) not null, primary key (cname, acctNo), foreign key (cname) references customer, foreign key (acctNo) references account) 11/8/2018 B.Ramamurthy

Cascading delete and update Create table depositor (cname char(20) not null, acctNo char(10) not null, primary key (cname, acctNo), foreign key (cname) references customer on delete cascade on update cascade, foreign key (acctNo) references account) 11/8/2018 B.Ramamurthy

Assertions An assertion is a predicate expressing a condition that the database should always satisfy. It is more global in scope than domain and referential integrity constraints. Format: Create assertion <assertion_name> check predicate 11/8/2018 B.Ramamurthy

Assertion : example Create assertion sum_constraint check (not exists (select * from branch where (select sum(amount) from loan where loan.bname = branch.bname) >= (select sum(amount) from account where loan.bname =branch.bname))) 11/8/2018 B.Ramamurthy

Assertion Semantics When an assertion is created system tests it for validity. If the assertion is valid any future modifications to the database will be allowed only if it does not violate the assertion. The example in last slide makes sure total loan amount is NOT > total balance in a branch involved in loans. 11/8/2018 B.Ramamurthy

Trigger A trigger is a statement that is executed automatically by the system as a side effect of modification to the database. To design a trigger: Specify conditions under which the trigger is to be executed. Specify actions to be taken when trigger is executed. Triggers are performance deflators. 11/8/2018 B.Ramamurthy

Trigger example define trigger overdraft on update of account T (if new T.balance < 0 then (insert into loan values (T.bname , T.acctNo, - new T.balance) insert into borrower (select cname,acctNo from depositor where T.acctNo = depositor.acctNo) update account S set S.balance = 0 where S.acctNo = T.acctNo)) 11/8/2018 B.Ramamurthy

Functional Dependencies Functional dependency is a an extension of the notion of “key”. Functional dependencies are constrains on a set of legal relations. Functional dependency for   R,   R,   Holds on R if for all pairs of tuples t1 and t2 if t1[] = t2[] then t1[] = t2[] 11/8/2018 B.Ramamurthy

Example A B C D a1 b1 c1 d1 a1 b2 c1 d2 a2 b2 c2 d2 a2 b3 c2 d3 A  C but not C  A ABD 11/8/2018 B.Ramamurthy

Functional dependency in Bank Example Branch-schema Bnamebranch-city Bname  assets Customer-schema Cname customer-city Cnamecustomer-street Loan-Schema loanNo amount loanNo bname Account-schema accntNobname acctNo balance Others : none 11/8/2018 B.Ramamurthy

Closure of a set of Functional Dependencies Given a set of functional dependencies determine all the set of all functional dependencies that are implied. Consider R = (A, B, C, G, H, I) and a set of functional dependencies F A  B A  C CG  H CG I B  H Determine the closure F denoted by F. 11/8/2018 B.Ramamurthy