Definition And Explaination:-
ALTER TABLE allows you to change the structure of an existing table. For example, you can add or delete columns, create indexes, change the type of existing columns, or rename columns or the table itself.You can use ALTER TABLE to modify the structure of a table that has not been added to a database.Alter table command allows
    * To add the field;
    * To delete the field out;
    * To change the DEFAULT value for any of the field;
    * To add or delete the table PRIMARY KEY;