Foreign Key holds the reference to another table column value.It is also known as references constraint.
A foreign key means that values in one table must also appear in another table.A references constraint is only applied at SQL ‘insert’ and ‘delete’ times.
A foreign key can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.

Syntax:

CREATE TABLE table_name
(column1 datatype null/not null,.....
CONSTRAINT foreignkey_column
FOREIGN KEY (column1,.. columnN)
REFERENCES Refering_table (column1,... columnN)
);

The following example illustrates the creation and use of Foreign key constraints.
Let us create an EMPLOYEE table which holds the employee information like name ,id and salary.
Here emp_id is the primary key column.

SQL> create table EMPLOYEE(
name VARCHAR2(50),
emp_id NUMBER,
salary NUMBER(8,2),
CONSTRAINT EMPLOYEE_ID_PK PRIMARY KEY (emp_id)
);

Table created

Now create an Employee address table which holds the address details of the employees.The address table is linked to EMPLOYEE table through the foreign key emp_id;

SQL> create table EMPLOYEE_ADDRESS(
address varchar2(200),
emp_id number,
CONSTRAINT EMP_ADD_FK FOREIGN KEY (emp_id)
REFERENCES EMPLOYEE(emp_id)
);

Table created

Now let us insert one record in EMPLOYEE table.

SQL> insert into EMPLOYEE values('ABCD',1,12345);
1 row inserted

Now let us add address for this particular employee in EMPLOYEE_ADDRESS table.

SQL> insert into EMPLOYEE_ADDRESS values('abcd efgh',1);
1 row inserted

Now while entering the data in EMPLOYEE_ADDRESS table the data should be already present in EMPLOYEE table.Otherwise foreign key violation will result.

SQL>; insert into EMPLOYEE_ADDRESS values('abcd efgh',8);
ORA-02291: integrity constraint (TEST.EMP_ADD_FK) violated - parent key not found

Also if we delete a row from the EMPLOYEE table and if a reference exists in EMPLOYEE_ADDRESS table oracle will throw an error showing reference integrity dependency.

SQL> delete from EMPLOYEE where emp_id =1;
ORA-02292: integrity constraint (TEST.EMP_ADD_FK) violated - child record found

Related Articles,ORA-02449: unique/primary keys in table referenced by foreign
ORA-02292: integrity constraint violated – child record found
ORA-02291: integrity constraint violated – parent key not Found
Oracle/PLSQL: Foreign Keys | Oracle Referential Integrity
Oracle/PLSQL: Composite Primary Key
Oracle/PLSQL: Primary Key and Composite Primary Key

Technorati Tags:
Foreign Keys, Oracle Referential Integrity