1. ALTER TABLE: The required ALTER TABLE keywords begin the DDL statement which allows you to modify an existing table.
2. tablename: The tablename must identify an existing table. You may have to qualify the tablename with its schema name, as in schema.tablename
3. ADD/MODIFY/DROP: The ADD, MODIFY, or DROP keywords specify the basic type of action you want to perform on the table.
4. action_specific_syntax: The action specific_syntax can include a range of options, based on how you wish to modify the existing table structure.