SQL Keys and Constraints Justin Maksim

Key Declaration Key constraint defined within the CREATE TABLE command Key can be declared using either the PRIMARY KEY keyword or the UNIQUE keyword. Can have only one PRIMARY KEY can have many attributes declared UNIQUE

Methods for Key Declaration Method 1: append the keywords PRIMARY KEY after the attribute i.e. CREATE TABLE Student ( studentID VARCHAR(8) PRIMARY KEY, name CHAR(30) );

Method 2: Add element PRIMARY KEY with a parenthesized list of the attribute(s) forming the primary key i.e. CREATE TABLE Student( studentID VARCHAR(8), name CHAR(30), PRIMARY KEY (studentID) ); Can have multiple attributes using this method PRIMARY KEY (studentID, name)

Method 3: Use the keyword UNIQUE. Can appear exactly where PRIMARY KEY can appear. i.e. CREATE TABLE Student( studentID VARCHAR(8), name CHAR(30), UNIQUE (studentID) ); OR CREATE TABLE Student( studentID VARCHAR(8) UNIQUE, name CHAR(30) ); OR UNIQUE ( studentID, name)

Enforcing Key Constraints SQL system will check key constraint only when an insertion or update to the relation occurs An index on the attribute(s) declared to be keys is important to enforce key constraint efficiently.

Referential Integrity and Foreign Keys Foreign Key  Referenced attribute(s) of second relation must be declared primary key for their relation  Any value appearing in an attribute of a foreign key must appear in the corresponding attribute of the second relation

Declaring Foreign Key Method 1: CREATE TABLE Student( studentID varchar(8), name char(30), record# INT REFERENCES Record(record#) ); Method 2: CREATE TABLE Student( studentID varchar(8), name char(30), record# INT, FOREIGN KEY record# REFERENCES Record(record#) );

Maintaining Referential Integrity Default Policy:  Reject changes Cascade Policy:  Update changes Set-Null Policy CREATE TABLE Student( studentID varchar(8), name char(30), record# INT REFERENCES Record(record#) ON DELETE SET NULL ON UPDATE CASCADE );

Constraints On the values of Attributes Limits the values for attributes Can be expressed:  A constraint on the attribute  A constraint on the domain

Not-Null Constraints record# INT REFERENCES Record(record#) NOT NULL  Can’t update to null  Can’t insert null value  Can’t use the set-null policy

Attribute-Based CHECK Constraints After defining attribute, can include the CHECK constraint i.e. gender CHAR(1) CHECK (gender IN (‘F’,’M’)),

Domain Constraints Can create a domain and assign a constraint to a domain i.e. CREATE DOMAIN GenderDomain CHAR(1) CHECK (VALUE IN (‘F’,’M’)); Then gender GenderDomain,

References First Course In Database Systems, Jeffery D. Ullman and Jennifer Widom,1997 Prentice Hall, Inc.