参数排序方法，只能作用于临时表，不能作用于join的结果。不然报错！！！
正确用法如下：
set @p = null; set @r = 0;
select record_id,doctor_name,doctor_guid, rank,create_date
from
(
SELECT
s.*,
if(@p=s.record_id,@r:=@r+1,@r:=1) as rank,
@p:=s.record_id
from
(
select a.record_id,a.doctor_name,a.doctor_guid,create_date,a.id
from hmcdss2.mt_patient_record_doctor a
join rp_update_gc_patient_info b on a.record_id=b.record_id
where doctor_type = 4 and source_type=1
)s
order by s.record_id,s.id desc
) t
where rank = 1
错误用法：
set @p = null; set @r = 0;
select
record_id, doctor_name, doctor_guid
from
(
select
a.record_id,a.doctor_name,a.doctor_guid,
if(@p=a.record_id,@r:=@r+1,@r:=1) as rank,
@p:=a.record_id
from hmcdss2.mt_patient_record_doctor a
join rp_update_gc_patient_info b on a.record_id=b.record_id
where doctor_type = 2
order by a.record_id,create_date desc
) t
where rank = 1