Constraints CSED421 Database Systems Lab

Constraints To define an integrity constraint (IC) –IC : condition that must be true for any instance of the database 5 types of constraints –NOT NULL constraints –UNIQUE constraints –PRIMARY KEY constraints –FOREIGN KEY constraints –CHECK constraints

Specification As part of the table definition e.g., CREATE TABLE orders ( order_id NUMBER PRIMARY KEY, price NUMBER NOT NULL, … ); CREATE TABLE ( [ CONSTRAINT ], … );

Specification (cont.) Alteration after the table definition e.g., ALTER TABLE orders ADD PRIMARY KEY (order_id); e.g., ALTER TABLE orders MODIFY ( price NOT NULL ); ALTER TABLE ADD[DROP] [ CONSTRAINT ] ; ALTER TABLE MODIFY ( [ CONSTRAINT ] );

NOT NULL Constraint Prohibits a database value from being null –null : either unknown or not applicable To satisfy a NOT NULL constraint, every row in the table must contain a value for the column CREATE TABLE student ( sid NUMBER NOT NULL, … … … ); ALTER TALBE student MODIFY sid NOT NULL;

UNIQUE Constraint Prohibits multiple rows from having the same value in the same column or combination of columns. But allows some values to be null. CREATE TABLE promotions ( promo_id NUMBER UNIQUE, … … … ); ALTER TABLE promotions ADD UNIQUE (promo_id); ALTER TABLE promotions MODIFY promo_id UNIQUE;

PRIMARY KEY Constraint Prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null. Combines a NOT NULL constraint and a unique constraint in a single declaration. CREATE TABLE locations ( location_id NUMBER PRIMARY KEY, … … … ); ALTER TABLE locations ADD PRIMARY KEY (location_id); ALTER TABLE locations MODIFY location_id PRIMARY KEY;

PRIMARY KEY Constraint (cont.) CREATE TABLE locations ( nameCHAR(20), addrCHAR(20), … …, CONSTRAINT name_addr PRIMARY KEY (name, addr) ); ALTER TABLE locations ADD CONSTRAINT name_addr PRIMARY KEY (name, addr)

FOREIGN KEY Constraint Requires values in one table to match values in another table. Also called referential integrity constraint CREATE TABLE enrolled ( sid CHAR(20), … … …, CONSTRAINT fk_enrolled FOREIGN KEY (sid) REFERENCES students(sid) ); CREATE TABLE enrolled ( sid CHAR(20) REFERENCES students(sid) … … … ); ALTER TABLE enrolled ADD CONSTRAINT fk_enrolled FOREIGN KEY (sid) REFERENCES students(sid); ALTER TABLE enrolled MODIFY sid REFERENCES students(sid);

FOREIGN KEY Constraint What if a student tuple is deleted… –NO ACTION (default) –CASCADE / SET NULL CREATE TABLE Enrolled( … … …, FOREIGN KEY (sid) REFERENCES Students(sid) ON DELETE CASCADE / SET NULL ); sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666'History105B sidnameloginagegpa EnrolledStudents

CHECK Constraint Requires a value in the database to comply with a specified condition CREATE TABLE emp ( … … …, age NUMBER CHECK (age > 0) ); ALTER TABLE emp ADD CHECK (age > 0); ALTER TABLE emp MODIFY age CHECK (age > 0);

Practice 1 1.Bank_Acct 와 Customer_Info 테이블을 생성 Customer_Info : name VARCHAR2(20), addr VARCHAR(60), phone CHAR(10), VARCHAR2(40) Bank_Acct : acct CHAR(4), name VARCHAR2(20), addr VARCHAR2(60), balance REAL Customer_Info’s primary key is (name, addr) Bank_Acct’s primary key is acct Bank_Acct’s (name, addr) references Customer_Info’s (name, addr)

Practice 1 2. 두 테이블에 대한 위의 인스턴스에 대해 오류 (IC) 를 찾기 3. 두 테이블에 각각의 레코드를 IC 를 만족하도록 필드 값을 수정하여 추가하기 NameAddrPhone Jones11 First Smith12 First Green14 Fourth Gates11 First Smith12 Second AcctNameAddrbalance 1005Jones11 First St Smith12 First St Smith12 First St Green14 Fourth St Gates20 Tenth St Smith15 Second St443 Bank_AcctCustomer_Info

Practice 1 4. 아래의 SQL 을 수행결과를 확인 SELECT B.name, C. FROM Bank_Acct B, Customer_Info C WHERE B.name = C.name AND B.addr = C.addr AND B.balance >

Practice 2 Simulate violation of integrity constraint 스크립트를 통해 테이블 생성. Practice2 스크립트는 Enrolled 와 Students 테이블 을 생성하고, Enrolled 의 Students 를 참조하는 foreign key constraint 를 추가한다. sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666'History105B sidnameloginagegpa EnrolledStudents

Practice 2 practice2.sql CREATE TABLE Students ( sid CHAR(20) PRIMARY KEY, name CHAR(20), login CHAR(10), age INTEGER, gpa REAL ); CREATE TABLE Enrolled ( sid CHAR(20), cid CHAR(20), grade CHAR(2), CONSTRAINT fk_enrolled FOREIGN KEY (sid) REFERENCES Students (sid) ); INSERT INTO Students VALUES ('53666', 'Jones', 18, 3.4); INSERT INTO Students VALUES ('53688', 'Smith', 18, 3.2); INSERT INTO Students VALUES ('53650', 'Smith', 19, 3.8); INSERT INTO Enrolled VALUES ('53666', 'Carnatic101', 'C'); INSERT INTO Enrolled VALUES ('53666', 'Reggae203', 'B'); INSERT INTO Enrolled VALUES ('53650', 'Topology112', 'A'); INSERT INTO Enrolled VALUES ('53666', 'History105', 'B');

Practice 2 2. 각각의 referential integrity enforcement 에 따라 Students 의 레코드를 삭제한다. (NO ACTION, CASCADE) 3. 결과 메시지 혹은 삭제 후 테이블의 전체 내용들 을 확인한다.

Practice 2 TAs want to see query results… –NO ACTION enforcement –CASCADE

Constraint Check Confirm constraints on the tables. –Use a system table, user_constraints. SQL> SELECT table_name, constraint_name, constraint_type FROM user_constraints WHERE table_name = ‘your table name’; SQL> SELECT table_name, constraint_name, constraint_type FROM user_constraints