Definition: When you are altering a table to add in another colum you can use Add Unique. This will add in a new column, and make sure that it's not a duplicate of any of the already existing columns. It is written as: alter table your_table add unique (column_name)
Also Known As: Add Unique Column
This code would add a new column, eye_color, to the table called people - if that column didn't already exist.
alter table people add unique (eye_color)