How to use sql ALTER TABLE statement

The ALTER TABLE statement is used to add, delete or modify an sql existing table.

To add a column in a table, use the following syntax:
    ALTER TABLE table_name
    ADD column_name datatype

To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):
    ALTER TABLE table_name
    DROP COLUMN column_name

To change the data type of a column in a table, use the following syntax:
    ALTER TABLE table_name
    ALTER COLUMN column_name datatype