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The ALTER TABLE statement allows to change the structure of a table after it has been created. New columns can be added with the ADD clause. Existing columns may be removed from a table using the Drop clause.
The syntax of the ALTER TABLE statement is as follows:
ALTER TABLE table_name
ADD column_name data_type
ALTER TABLE table_name
DROP COLUMN column_name
Note: The ALTER TABLE statement is not part of ANSI/ISO standard. This means that all the commercial dialects of SQL implement different clauses and command syntax.
Example #1
ALTER TABLE Orders
ADD Quantity SMALLINT
This example adds a single column to the "Orders" table by specifying the new column name and data type.
Example #2
ALTER TABLE EmployeeAddressTable
ADD Description VARCHAR(20) DEFAULT 'USA' NOT NULL
This example adds a column named "Description" to the "EmployeeAddressTable" table. We also add the reserved words NOT NULL to require valid data to be added to that column and specify a default value.
Example #3
ALTER TABLE AntiqueOwners DROP COLUMN State
This example drops the "State" column from the "AntiqueOwners" table. | 
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