Primary Key

Primary keys are the unique identifiers for each row. They must contain unique values (and hence cannot be NULL). Because of their importance in relational databases 
Primary keys are the most fundamental of all keys and constraints. A table can have a maximum of one primary key. A primary key can consist of one or more fields on a table. When multiple fields are used as a primary key, they are called a composite key.


create table employee
{
empid int primary key,
empname varchar(20),
empLname varchar(20),
}


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