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computer network is system in which multiple computer devices are connected to each other to share information and resources  LAN- local area network is computer network that interconnects computer devices within limited area such as school, laboratery . Man- metropolitan area like city . wan- large scale geographical area. protocol- is set of rules to govern data communication  OSI MODEL=  OSI stands for  Open System Interconnection   is a reference model that describes how information from a   software   application in one   computer   moves through a physical medium to the software application in another computer. Internet - is type of world wide computer network  internet is computer network that interconnects hundreds of millions of computer devices throughout the world  WEB-  Web page consist of objects . object can be HTML file,text file, audio file etc.. if webpage consist of HTML text and five images then web page has ...

 Primary key CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID) ); CREATE TABLE CUSTOMERS( ID INT NOT NULL, NAME VARCHAR (20) NOT NULL, AGE INT NOT NULL, ADDRESS CHAR (25) , SALARY DECIMAL (18, 2), PRIMARY KEY (ID, NAME) ); ALTER TABLE CUSTOMER ADD PRIMARY KEY (ID); ALTER TABLE CUSTOMERS ADD CONSTRAINT PK_CUSTID PRIMARY KEY ( ID , NAME ); ALTER TABLE CUSTOMERS DROP PRIMARY KEY ; foreign key CUSTOMERS table CREATE TABLE CUSTOMERS ( ID INT NOT NULL , NAME VARCHAR ( 20 ) NOT NULL , AGE INT NOT NULL , ADDRESS CHAR ( 25 ) , SALARY DECIMAL ( 18 , 2 ), PRIMARY KEY ( ID ) ); ORDERS table CREATE TABLE ORDERS ( ID INT NOT NULL , DATE DATETIME , CUSTOMER_ID INT refer...