Modify Columns

Use alter table to modify an existing column. You can modify any number of columns in a single alter table statement.

For example, this command changes the datatype of the type column in the titles table from char(12) to varchar(20) and makes it nullable:

alter table titles
modify type varchar(20) null
Warning!   You may have objects (stored procedures, triggers, and so on) that depend on a column having a particular datatype. Before you modify a column, use sp_depends to determine a table’s dependent objects, and to make sure that any objects that reference these objects can run successfully after the modification.