SQL Server Primary Key

Whats is primary Key in SQL | How to create Primary Key in SQL | Primary Key with Example in SQL Server

The PRIMARY KEY constraint which is uniquely identifies each row in a table into the Database. It must contain UNIQUE values and they can not be null.A table in SQL is strictly restricted to have one and only one primary key, which is comprised of single or multiple fields (columns).
CREATE TABLE Employee ( /* Create table with a single field as primary key */    ID INT NOT NULL    Name VARCHAR(255)    PRIMARY KEY (ID));
CREATE TABLE Employees ( /* Create table with multiple fields as primary key */    ID INT NOT NULL    LastName VARCHAR(255)    FirstName VARCHAR(255) NOT NULL,    CONSTRAINT PK_Employee    PRIMARY KEY (ID, FirstName));
ALTER TABLE Employees /* Set a column as primary key */ADD PRIMARY KEY (ID);
ALTER TABLE Employees /* Set multiple columns as primary key */ADD CONSTRAINT PK_Employee /*Naming a Primary Key*/PRIMARY KEY (ID, FirstName);