SQL Server 2008 allows existing tables to be modified in several ways. Using the ALTER TABLE command, you can make the following types of changes to an existing table:
Change the data type or the NULL property of a single column.
Add one or more new columns, with or without defining constraints for those columns.
Add one or more constraints.
Drop one or more constraints.
Drop one or more columns.
Enable or disable one or more constraints (applies only to CHECK and FOREIGN KEY constraints).
Enable or disable one or more triggers.
Change the lock escalation behavior ...