An Identity column in SQL-SERVER It is automatically inserted the value in identity column whenever new record gets inserted into the table. --Create Student if object_id ( 'Student' ) is null create table Student ( id tinyint identity ( 1 , 1 ) , Name varchar ( 20 ), Marks int ) If we observe above Create table statement the Id column is identity column and its data type is tinyint . We know that tinyint accepts 0 to 255 numbers range. --insert data into Student table. Declare @start int = 1 while ( @start <= 256 ) begin insert into Student ( Name , Marks ) values ( 'Rakesh' , 500 ) set @start = @start + 1 end In the above while loop we are trying to insert same record in 256 time. The identity column automatically supplied value. The 1-255 records gets inserted successfully.256 record get an error because of tinyint accepts 0-255 records ...