CONSTRAINTS ???

Can someone tell how to use diff Constraints like primary key , unique , foreign key etc ... with some examples......

Thanx in advance....

Few Examples

PRIMARY KEY CONSTRAINT

Code:

ALTER TABLE STUDENT_DETAILS ADD CONSTRAINT <constraint_name> PRIMARY KEY(ROLLNO);

FOREIGN KEY

Code:

ALTER TABLE STUDENT_MARKS_HISTORY ADD CONSTRAINT <constraint_name> FOREIGN KEY (ROLLNO) REFERENCES STUDENT_DETAILS(ROLLNO);

UNIQUE KEY

Code:

ALTER TABLE STUDENT_DETAILS ADD CONSTRAINT <constraint_name> UNIQUE(NAME);

Thanx.........

these are applicable only after creating table rite???

how to make some field as a foreign key , while creating a table ?

@praveen

the column name should be same and use the keyword REFERENCES
eg. let there is table emp(empid,empname)

now to create foreign key in dept table v can use
dept(deptid, empid number references emp)

and the referenced column should be primary key.

@parwani_ravi

Thank u ........

its not necessary tat foreign key has to be in the same name.

Code:


CREATE TABLE
          STUDENT_MARK_HISTORY (RNO NUMBER(4)
                          CONSTRAINT <constraint_name> REFERENCES STUDENT_DETAILS(ROLLNO),
                                      NAME VARCHAR2(20), AGE NUMBER(2))