The syntax to modify a column to include user-defined datatypes is the same as modifying a column to include system-defined datatypes. For example, to modify the au_lname of the authors table to use the user-defined newtype datatype:
alter table authors modify au_lname newtype(60) not null
If you do not specify either NULL or NOT NULL as the default, columns use the default specified by the user-defined datatype.
Modifying the table does not affect any current rules or defaults bound to the column. However, if you specify new rules or defaults, any old rules or defaults bound to the user-defined datatype are dropped. If there are no previous rules or defaults bound to the column, any user-defined rules and defaults are applied.
You cannot modify an existing column to an IDENTITY column. You can only modify an existing IDENTITY column with user-defined datatypes that have IDENTITY column properties (precision and scale).
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