Constraints

Constraints

CSED421Database 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 <table_name> (<column_name> <type>

[ CONSTRAINT <constraint_name> ] <condition>,…

);

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 <table_name>ADD[DROP] [ CONSTRAINT <constraint_name> ] <condition>;

ALTER TABLE <table_name>MODIFY (

<column_name> [ CONSTRAINT <constraint_name> ] <condition>);

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 promotionsADD UNIQUE (promo_id);

ALTER TABLE promotionsMODIFY 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 locationsADD PRIMARY KEY (location_id);

ALTER TABLE locationsMODIFY location_id PRIMARY KEY;

PRIMARY KEY Constraint (cont.)

CREATE TABLE locations (name CHAR(20),addr CHAR(20),… …,CONSTRAINT name_addr PRIMARY KEY (name,

addr));

ALTER TABLE locationsADD 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 constraintCREATE 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 enrolledADD CONSTRAINT fk_enrolled FOREIGN KEY (sid)

REFERENCES students(sid);

ALTER TABLE enrolledMODIFY 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 );

sid cid grade

53666

Carnatic101 C

53666

Reggae203 B

53650

Topology112

A

53666

'History105 B

sid name

login age gpa

53666

Jones jones@cs 18 3.4

53688

Smith smith@eecs 18 3.2

53650

Smith smith@math

19 3.8

Enrolled Students

CHECK Constraint

• Requires a value in the database to comply with a specified condition

CREATE TABLE emp (… … … ,age NUMBER CHECK (age > 0)

);

ALTER TABLE empADD CHECK (age > 0);

ALTER TABLE empMODIFY 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), email 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

Name Addr Phone Email

Jones 11 First St 603-222-3456

Jones@cs

Smith 12 First St 603-333-5567

Smith@cs

Green 14 Fourth St 781-444-7890

Green@cs

Gates 11 First St 122-333-6789

Gates@ms

Smith 12 Second St 603-978-8765

Smith@ee

Acct Name Addr balance

1005

Jones 11 First St 1023

1002

Smith 12 First St 22

1003

Smith 12 First St 1100

1008

Green 14 Fourth St 1077

1010

Gates 20 Tenth St 50000

1012

Smith 15 Second St 443

Bank_AcctCustomer_Info

Practice 1

4. 아래의 SQL 을 수행결과를 확인

SELECT B.name, C.emailFROM Bank_Acct B, Customer_Info CWHERE B.name = C.name AND B.addr = C.addr AND B.balance > 1000.00

Practice 2

• Simulate violation of integrity constraint1. @practice2 스크립트를 통해 테이블 생성 .• Practice2 스크립트는 Enrolled 와 Students

테이블을 생성하고 , Enrolled 의 Students 를 참조하는 foreign key constraint 를 추가한다 .

sid cid grade

53666

Carnatic101 C

53666

Reggae203 B

53650

Topology112

A

53666

'History105 B

sid name

login age gpa

53666

Jones jones@cs 18 3.4

53688

Smith smith@eecs 18 3.2

53650

Smith smith@math

19 3.8

Enrolled Students

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', 'jones@cs', 18, 3.4);INSERT INTO Students VALUES ('53688', 'Smith', 'smith@eecs', 18, 3.2);INSERT INTO Students VALUES ('53650', 'Smith', 'smith@math', 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 enforcement에 따라 Students 의 레코드를 삭제한다 .(NO ACTION, CASCADE)

3. 결과 메시지 혹은 삭제 후 테이블의 전체 내용들을 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