Changes an existing table definition. You can:

    o  Add columns

    o  Add constraints to tables or columns

    o  Modify columns

    o  Modify character sets

    o  Modify data types

    o  Delete columns

    o  Delete constraints

    The ALTER TABLE statement can also add or delete table-specific
    constraints. You can display the names for all constraints
    currently associated with a table by using the SHOW TABLE
    statement. Any number of constraints can be deleted and
    declared at both the table and column levels. See also the ALTER
    CONSTRAINT statement and the DROP CONSTRAINT. statement.

    When you execute this statement, SQL modifies the named column
    definitions in the table. All of the columns that you do not
    mention remain unchanged. SQL defines new versions of columns
    before defining constraints. Then, SQL defines and evaluates
    constraints before storing them. Therefore, if columns and
    constraints are defined in the same table definition, constraints
    always apply to the latest version of a column.

    When you change a table definition, other users see the revised
    definition only when they connect to the database after you
    commit the changes.