The definition of a base table can be changed by using the ALTER TABLE
command, which is a schema evolution command. The possible alter table
actions include adding or dropping column(attribute), changing a column
definition, and adding or dropping table constraints. For example, to
add an attribute for keeping track of job of employees to the EMPLOYEE
base relation in the COMPANY schema, we can use the command:

        ALTER TABLE COMPANY.EMPLOYEE AND JOB VARCHAR(12);