DATABASE (SQL)day 3) CHECK CONSTARAINT , KEY, DROP COLUMN(DELETE)

                                                      * CHECK CONSTRAINT *

CHECK CONSTARINT DIRECTRY APPLY TO PERTICULAR COLUMN.

QUERY:-

 - create table emp(id int ,name varchar(50),salary INT ,CONSTRAINT sell CHECK(salary>1000));

  -    INSERT INTO emp VALUES (101,'MOHAN',2000);

  -   INSERT INTO emp VALUES (102,'SOHAM',500);

   -  SELECT * FROM emp;


                                              *KEY*

* KEY  IS USE TO MANAGING THE TABLE .

*THERE ARE SOMETYPES OF KEY

 1)PRIMARY KEY  :-NO BLANK ,NO REPEAT

 2)UNIQUE KEY     :-NO REPEAT ,BLANK POSSIBLE

 3)FOREIGN KEY

                  *PRIMARY KEY*

Q)TYPES OF METHOD IS USED TO CREATING A KEY.

 1)COLOUM LEVEL PRIMARY KEY:-1COLUNM

2)TABEL LEVEL PRIMARY KEY :-MORE THAN COLUNM

*-QUERY-*

CREATE TABLE stu2(roll INT PRIMARY KEY ,name VARCHAR (50));

INSERT INTO stu2 VALUES (201,'SONAM');

SELECT * FROM stu2;


                                    *UNIQUE KEY*

CREATE TABLE stu3(roll INT UNIQUE,name VARCHAR(50));

INSERT INTO stu3 VALUES(301,'AJAY');

SELECT * FROM stu3;

  

                                       *DROP COLUMN*

Q) HOW TO DELETE COLUMN

= ALTER COMMAND AND DROP COLUMN COMMAND.

QUERY:-

ALTER TABLE tablename DROP COLUMN columnname;


check constraint ,key,drop delete column 










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