Sometimes we may need to make a column unique in a database table. For instance we can't have two employees with the same employees with the same employee number or pension plan id.
To prevent this, run a query like the following to place a constraint on the attribute or attributes;
ALTER TABLE Employee ADD UNIQUE (EmpID, PensionID) |
There are many other uses of the ALTER command that will help us modify our existing tables. See this article for more useful applications of the ALTER command.