In SQL Server, it's called an Identity column.
To make an existing column an Identity...
Open SQL Server Management Studio
In Object Explorer, on the left, drill down in to your table.
Right click the table, and click Design.
Click on the column you want to be an Identity
At the bottom of the window, you will see "Column Properties"
Scroll down and expand "Identity Specification"
Set (Is Identity) = Yes
You can change the Increment and Seed if you want to.
Increment is just that. How much to increment each value by. This is normally 1, but doesn't have to be.
Seed is the starting value. Again, this is normally 1, but doesn't have to be.
-George
Microsoft SQL Server MVP
My Blogs
SQLCop
"The great things about standards is that there are so many to choose from." - Fortune Cookie Wisdom