添加列前判断

drop procedure if exists schema_change;  
delimiter ';;';  
create procedure schema_change() begin 
if exists (select * from information_schema.columns where table_name = 'sales_order' and column_name = 'has_sent')

then 
        alter table sales_order drop column has_sent;  
end if;  
alter table sales_order add column has_sent boolean;  
end;;  

delimiter ';';  
call schema_change();  
drop procedure if exists schema_change;