sql - Max Function of Mysql not returning maximum value of coloumn. Order by limit also not working -

i have table structure follows:

                     history_card      +----+-----------+---------+--------------+      | idhistory_card |equipment_id | sr_num |          +----+-----------+---------+--------------+      |  1             |      du 201 | 9      |      |  2             |      du 201 | 2      |      |  3             |      du 201 | 12     |      |  4             |      ext 99 | 10     |      |  5             |      ext 99 | 13     |      |  6             |      ext 99 | 7      |      |  7             |      ext 99 | 9      |       +----+-----------+---------+--------------+ 

i return maximum value of coloumn(sr_num) particular euipment_id. eg. equipment_id = du 201 , maximum value 12 below mentioned queries returning wrong value i.e 9 equipment_id = ext 99 , maximum value 13 below mentioned queries returning wrong value i.e 9 . have searched lot in internet on manual of no use till now.kindly help!

queries follows

select * new_schema.history_card equipment_id = 'du 201' order sr_num desc limit 1 

2 query tried

select max(sr_num) new_schema.history_card equipment_id='du 201'; 

the max() operation on string value behave according string comparison rules, i.e. "9" > "10".

the answer change table definition:

alter table history_card modify sr_num int; 

then, run query again.