how do Primary keys work?

Answer 1

create table studenttable (st.name varchar (10) ,rollno int primary key);

//i have applied primary key on field/column/key " rollno " , u cannot left this column blank
//here u cannot add more than 1 record which have same roll no.
// actually primary key is used to identify records uniquely
//here each records gets unquely idetified as every student have different rollno. in records of table studenttable