A composite key can be defined on a
combination of columns.
We can define composite keys on entity
integrity and referential integrity constraints.
Composite key can be defined in table and
alter levels only.
Ex:
UNIQUE (TABLE LEVEL)
SQL> create table student12(no number(2) , name varchar(10), marks
number(3),
unique(no,name));
SQL> create table student12(no number(2) , name varchar(10), marks
number(3),
constraint un unique(no,name));
UNIQUE (ALTER
LEVEL)
SQL> alter table student12 add unique(no,name);
SQL> alter table student12 add constraint un unique(no,name);
PRIMARY KEY
(TABLE LEVEL)
SQL> create table student12(no number(2) , name varchar(10), marks
number(3),
primary key(no,name));
SQL> create table student12(no number(2) , name varchar(10), marks
number(3),
constraint pk primary key(no,name));
PRIMARY KEY (ALTER LEVEL)
SQL> alter table student12 add primary key(no,anme);
SQL> alter table student12 add constraint pk primary key(no,name);
FOREIGN KEY (TABLE LEVEL)
SQL> create table emp(empno number(2), ename
varchar(10), deptno number(2),
dname varchar(10), primary
key(empno), foreign key(deptno,dname) references
dept(deptno,dname));
SQL> create table emp(empno number(2), ename
varchar(10), deptno number(2),
dname varchar(10), constraint pk
primary key(empno), constraint fk foreign
key(deptno,dname) references dept(deptno,dname));
FOREIGN KEY (ALTER LEVEL)
SQL> alter table emp add foreign key(deptno,dname) references
dept(deptno,dname);
SQL> alter table
emp add constraint fk foreign key(deptno,dname) references
dept(deptno,dname);
