You can add a primary key to a table during or after table creation if the primary key does not already exist. If columns that are part of a primary key do not have nullability defined, the Database Engine automatically configures them as NOT NULL.   SQL Server supports many ways to define a primary key when creating a table. If you want to define a primary key on only one column, you can include it as part of the column definition as shown in the following example.    CREATE TABLE Students
(
studentId INT PRIMARY KEY,
studentName VARCHAR(20) NOT NULL
);  In the above example, the table named as Students is being created using SQL statements. In this table studentId column is defined as primary key.