Use the ALTER TABLE statement to modify a table definition by adding, altering, or dropping columns and constraints.
Despite the SQL standard, the implementation of ALTER TABLE varies greatly by DBMS. To determine what you can alter and the conditions under which alterations are permitted, search your DBMS documentation for ALTER TABLE. Depending on your DBMS, some of the modifications that you can make by using ALTER TABLE are:
Add or drop a column
Alter a column’s data type
Add, alter, or drop a column’s nullability or default constraint
Add, alter, or drop column or table constraints such as primary-key, foreign-key, ...