DataBase Constraint

 l DataBase Constraint:-

I. Data base constraint are the rules apply for the column in the database.

l Types od database constraint:-

1. Primary key constraint

2. Unique key constraint

3. Forigen key constraint

4. Not null constraint

5. Defult constraint

6. Check constraint :-

Syntax :- create table tablename(columnname datatype , CONSTRAINT name CHECK (CONDITION) );

Ex:-CREATE TABLE QDB(ID INT , AGE INT  CONSTRAINT  RST  CHECK (Age>20));

 

l How to apply constraint in existing table?

Ø Syntax:- ALTER TABLE TABLENAME ADD CONSTRAINT CONSTRAINTNAME CHECK (COND);

EX:-CREATE TABLE RK (ID INT , AGE INT );

ALTER TABLE RK ADD CONSTRAINT MY CHECK(AGE>20);

INSERT INTO RK VALUES (2,265);

l HOW TO REMOVE CONSTRAINT?

Ø SYNTAX:- ALTER TABLE TABLENAMR DROP CONSTRAINT

CONSTRAINTNAME ;

Ex:- ALTER TABLE RK DROP CONSTRAINT MY ;

 

INSERT INTO  rk VALUES(200,8);

 

 

 

 

l Default constraint:-default constraint assigned a default value to a column.

l Default constraint is uses defult keywords.

l Ex:-CREATE TABLE DEFDB(ID INT ,CITY VARCHAR(50) DEFAULT 'NAGPUR'); 

l insert into defdb values (101,‘pune’);

l   Task 3 create default constraint using alter command

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