The AUTOINCREMENT default is useful for numeric data fields where the value of the number itself may have no meaning. The feature assigns each new row a unique value larger than any other value in the column. You can use AUTOINCREMENT columns to record purchase order numbers, to identify customer service calls or other entries where an identifying number is required.
Autoincrement columns are typically primary key columns or columns constrained to hold unique values (see Enforcing entity integrity).
You can retrieve the most recent value inserted into an autoincrement column using the @@identity global variable. For more information, see @@identity global variable.
Autoincrement is intended to work with positive integers.
The initial autoincrement value is set to 0 when the table is created. This value remains as the highest value assigned when inserts are done that explicitly insert negative values into the column. An insert where no value is supplied causes the AUTOINCREMENT to generate a value of 1, forcing any other generated values to be positive.
A column with the AUTOINCREMENT default is referred to in Transact-SQL applications as an IDENTITY column.
For information on IDENTITY columns, see The special IDENTITY column.