mysql case

查看粉丝 及是否关注该粉丝
select f.*, case when f1.id is null then '1' else '0' end
f_state from follow f left join follow f1 
on f1.follow_from = f.follow_to 
and f1.follow_to = f.follow_from and f1.state = '0'
where f.follow_to = 2 and f.state = '0'