mysql中update case when的使用

    set @beginTime='2019-12-07 00:00:00';
  set @endTime='2020-01-05 23:59:59';

update users_jianfu as a inner join (select user_id,user_order_num from user_orders_last30days_temp order by user_id asc  limit 850000,10000) as b  on a.user_id=b.user_id

set a.activity_id=(
case 
when a.register_time<@beginTime  and b.user_order_num>=1 and  b.user_order_num<=3 then 10
when a.register_time<@beginTime  and b.user_order_num>3 then 20
when a.register_time>=@beginTime and a.register_time<=@endTime then 30
else 0 
end
)