ALTER TABLE statement

Use the ALTER TABLE statement to change the structure of an existing table in a database file. You can modify only one column in each statement. The formats of the ALTER TABLE statement are:

Copy

ALTER TABLE table_name ADD [COLUMN] column_definition
ALTER TABLE table_name DROP [COLUMN] unqualified_column_name
ALTER TABLE table_name ALTER [COLUMN] column_definition SET DEFAULT expr 
ALTER TABLE table_name ALTER [COLUMN] column_definition DROP DEFAULT

You must know the table’s structure and how you want to modify it before using the ALTER TABLE statement.

Examples

To	Sample SQL
Add columns	Copy `ALTER TABLE Salespeople ADD C1 VARCHAR`
Remove columns	Copy `ALTER TABLE Salespeople DROP C1`
Change a table name	Copy `ALTER TABLE Salespeople RENAME TO Salesperson`
Change a column name	Copy `ALTER TABLE Salespeople RENAME COLUMN Company TO Corporation`
Set the default value for a column	Copy `ALTER TABLE Salespeople ALTER Company SET DEFAULT 'Claris'`
Remove the default value for a column	Copy `ALTER TABLE Salespeople ALTER Company DROP DEFAULT`

Note SET DEFAULT and DROP DEFAULT do not affect existing rows in the table, but change the default value for rows that are subsequently added to the table.