Altering tables and contraints

Once you have created your tables, you can modify their structure by using the ALTER TABLE command. Add columns, modify the column length, add or drop constraints, and enable or disable constraints by using this command. If you want to remove a table, both the rows and the data structure of a table, invoke the DROP TABLE command. Other commands that affect tables that are covered in this lesson are
RENAME, to change a database object name.
TRUNCATE, to remove all rows from a table.
COMMENT, to add a comment about a database object to the data dictionary. All of these commands are data definition commands...