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Hi Friends,

Let's give a brief light on Constraints

Constraint:- It is the rule which restrict the incorrect data to be inserted into the database.

Types of Constraint:-

1. Not Null Constraint

2. Unique Constraint

3. Check Constraint

4. Primary Key Constraint

5. Foreign Key Constraint

6. Ref Key(Referential Kay) Constraint

TABLE LEVEL CONSTRAINT:-

COLUMN LEVEL CONSTRAINT:-

1. NOT NULL:-This is the constraints will avoid null values to the column specified.

NOTE:- It is the only constraint cannot be defined at Table level.

Syntax:-Create table <table_name>

(Empno NUMBER Constraint Empno_NN NOT NULL,

ENAME VARCHAR2(30) NOT NULL

);

SQL> create table emp_info

2 (

3 empno number Constraint empno_pk Primary key,

4 ename varchar2(20) Not Null,

5 Job varchar2(20) Constraint job_NN Not Null,

6 MGR NUMBER Constraint mgr_ref References emp_info(empno),

7 HIREDATE Date,

8 Sal Number Constraint sal_chk CHECK (sal Between 500 and 5500),

9 comm Number,

10 Deptno Number,

11 Constraint deptno_fk FOREIGN KEY(deptno) References dept(deptno)

12 );

Data Dictionary:-User_Constraints , User_cons_columns