The Alter SQL keyword is primarily used to modify the structure of a table in your database.   

ADD COLUMN

     ex.: 

        Alter table tablename ADD Column column_name varchar(40)

CHANGE COLUMN

     ex.: 

         Alter table tablename Change Column column_name1 column_name2 varchar(40)

DROP COLUMN

     ex.: 

        Alter table tablename Drop Column column_name

ADD INDEX

     ex.: 

        Alter table tablename Add Index indexname (column_name)

DROP INDEX

     ex.: 

        Alter table tablename Drop Index index_name

RENAME AS

     ex.: 

       Alter table tablename Rename As new_tablename