cheat sheets.

add a column:
ALTER TABLE table_name ADD column_name datatype
E.g: 
  alter table books add total_page_num float default 100 NOT NULL

drop a column:
ALTER TABLE table_name drop Gender column_name

modify datatype of column:
ALTER table table_name modify column_name datatype

completely change a column
ALTER table table_name change column_name new_column_name attributes

add an index
  | ADD {INDEX|KEY} [index_name]
        [index_type] (index_col_name,...) [index_option] ...
E.g:
  alter table track_offer_clicks add index fubar1 (application_id,
  landing_page_id );