使用timestampdiff函数：

select       
t.userid,     
case when t.years > 0 
     then concat( t.years ,' year', case when t.years > 1
                                         then 's' else '' end, ' ago')  
     when t.months > 0         
     then concat( t.months, ' month', case when t.months > 1    
                                           then 's' else '' end, ' ago')
     when t.weeks > 0 
     then concat( t.weeks , ' week', case when t.weeks > 1 
                                          then 's' else '' end, ' ago')  
     when t.days > 0  
     then concat( t.days , ' day', case when t.days > 1 
                                        then 's' else '' end, ' ago')  
     when t.hours > 0 
     then concat( t.hours, ' hour',  case when t.hours > 1 
                                          then 's' else '' end, ' ago')  
     when t.minutes > 0
     then concat( t.minutes, ' minute', case when t.minutes > 1  
                                          then 's' else '' end, ' ago')    
     else concat( t.seconds, ' second', case when t.seconds > 1
                                          then 's' else '' end, ' ago')
end as last_login  
from  
(  
select   
    userid,   
    timestampdiff(second, login_time, now()) as seconds,  
    timestampdiff(minute, login_time, now()) as minutes,  
    timestampdiff(hour, login_time,now()) as hours,  
    timestampdiff(day, login_time,now()) as days,  
    timestampdiff(week, login_time,now()) as weeks,  
    timestampdiff(month, login_time, now()) as months,  
    timestampdiff(year, login_time, now()) as years 
from user_login ) t

sqlfiddle:http://sqlfiddle.com/#!9/a2cbee/5号