在MySQL表中添加索引？

Question

非常基本的问题 - 如何为MySQL表的列添加索引？

CREATE TABLE housekeeper_stats 
(company_site_id int,    
company_id int,    
event_count int,    
host varchar(100),    
source varchar(100),    
source_type varchar(100));

mysql> alter table housekeeper_stats add index (company_id);
ERROR 1072 (42000): Key column 'company_id' doesn't exist in table

mysql> create index company_idx_n1 on housekeeper_stats(company_id, company_site_id) using btree;

ERROR 1072 (42000): Key column 'company_id' doesn't exist in table


The documentation states:

ADD {INDEX|KEY} [index_name]
        [index_type] (index_col_name,...) [index_option] 

不知道我哪里错了？

感谢。