Mysql 组合来自不同表的2个SQL查询

我有一个预约数据库，包含医生和过去的预约。 当用户希望预约医生时，他会选择一个住院医师，之后会根据该住院医师向他展示医生。现在是棘手的部分：

医生们将按照过去预约的顺序进行展示，这意味着如果我过去拜访过两位心脏病专家，我预约的最后一位将首先展示，在他之后是另一位，在他们之后是我从未预约过的所有其他心脏病专家

为此，我需要查找我过去与心脏病专家的预约，并按日期降序排列，然后去找所有医生，只需将其他医生添加到列表的底部

我正在尝试以下查询：

(
select distinct pastappointments.doctorID from pastappointments where
     pastappointments.insuredID = 1  and pastappointments.residency='cardio'
order by pastappointments.appTime desc
)
union (
select employees.employeeID from employees where  
employees.Residency='cardio'  and employees.employeeID not in (
select distinct pastappointments.doctorID from pastappointments where  
pastappointments.insuredID = 1  and pastappointments.residency='cardio'
))

我遇到的问题主要是在union命令上方运行第一个查询时，如果没有括号，我将获得正确的顺序，一旦括号，查询结果将更改为错误的顺序

我正在使用MySQL

---

我希望使用一个查询而不是两个不同的查询来执行此操作，但我似乎无法找到一种方法来执行此操作而不丢失所需的降序。

如果希望结果按特定顺序排列，则需要在最外层的查询中指定

order by

在MySQL中，这是在

联合之后。我还强烈建议使用
union all
而不是
union
，除非您明确希望承担删除重复项的开销：

(select pa.doctorID, max(pa.appTime) as last_apptime
 from pastappointments pa
 where pa.insuredID = 1 and pa.residency = 'cardio'
 group by pa.doctorID
) union all
(select e.employeeID, NULL as last_apptime
 from employees e
 where e.Residency = 'cardio' and
       e.employeeID not in (select pa.doctorID
                            from pastappointments pa
                            where pa.insuredID = 1 and       
                                  pa.residency = 'cardio'
                           )
)
order by (last_apptime is not null) desc, last_apptime desc;

这将返回两列。如果需要一列，只需使用子查询：

select t.doctorID
from (<above query without orderby) t
order by (last_apptime is not null) desc, last_apptime desc;

这还假设
e.residentity
与
pa.residentity
相同。如果不是这样，则需要
或
条件

这是编写查询的一种更合理的方法（假设所有医生都是员工）。
如果希望结果按特定顺序排列，则需要在最外层的查询中指定
order by

在MySQL中，这是在
联合之后。我还强烈建议使用
union all
而不是
union
，除非您明确希望承担删除重复项的开销：

(select pa.doctorID, max(pa.appTime) as last_apptime
 from pastappointments pa
 where pa.insuredID = 1 and pa.residency = 'cardio'
 group by pa.doctorID
) union all
(select e.employeeID, NULL as last_apptime
 from employees e
 where e.Residency = 'cardio' and
       e.employeeID not in (select pa.doctorID
                            from pastappointments pa
                            where pa.insuredID = 1 and       
                                  pa.residency = 'cardio'
                           )
)
order by (last_apptime is not null) desc, last_apptime desc;

这将返回两列。如果需要一列，只需使用子查询：

select t.doctorID
from (<above query without orderby) t
order by (last_apptime is not null) desc, last_apptime desc;

这还假设
e.residentity
与
pa.residentity
相同。如果不是这样，则需要
或
条件

这是编写查询的一种更合理的方法（假设所有医生都是员工）。
您必须用外部查询包围这两个查询，并对其应用order by

SELECT * FROM (
    SELECT DISCTINCT pastappointments.doctorID 
    FROM pastappointments 
    WHERE pastappointments.insuredID = 1 
    AND pastappointments.residency='cardio')
   UNION 
    (SELECT employees.employeeID 
     FROM employees 
     WHERE employees.Residency='cardio' 
     AND employees.employeeID NOT IN
       (SELECT DISTINCT pastappointments.doctorID 
        FROM pastappointments 
        WHERE pastappointments.insuredID = 1 
        AND pastappointments.residency='cardio'))
  )
 ORDER BY pastappointments.appTime DESC

您必须用外部查询包围这两个查询，并对其应用排序依据

SELECT * FROM (
    SELECT DISCTINCT pastappointments.doctorID 
    FROM pastappointments 
    WHERE pastappointments.insuredID = 1 
    AND pastappointments.residency='cardio')
   UNION 
    (SELECT employees.employeeID 
     FROM employees 
     WHERE employees.Residency='cardio' 
     AND employees.employeeID NOT IN
       (SELECT DISTINCT pastappointments.doctorID 
        FROM pastappointments 
        WHERE pastappointments.insuredID = 1 
        AND pastappointments.residency='cardio'))
  )
 ORDER BY pastappointments.appTime DESC

您可以使用左联接，并将缺少的约会时间替换为最后订购的值

select employees.employeeID 
from employees
left join pastappointments
  on  pastappointments.insuredID = 1
  and pastappointments.residency='cardio'
  and pastappointments.doctorID = employees.employeeID
group by employees.employeeID
where employees.Residency='cardio'
order by coalesce(max(pastappointments.appTime), cast(0 as datetime)) desc

您可以使用左联接，并将缺少的约会时间替换为最后订购的值

select employees.employeeID 
from employees
left join pastappointments
  on  pastappointments.insuredID = 1
  and pastappointments.residency='cardio'
  and pastappointments.doctorID = employees.employeeID
group by employees.employeeID
where employees.Residency='cardio'
order by coalesce(max(pastappointments.appTime), cast(0 as datetime)) desc

嗨，谢谢你的回复！你的第一个建议有效，但当我尝试使用更简单的建议时，它通知我一个意外的“位置”。我还有一个小问题，我可以在使用主查询时使用子查询吗？这样我从数据库得到的只是医生的ID号？您缺少左JOIN的ON子句（
pa.doctorID=e.employeeID
）。@PaulSpiegel。谢谢。嗨，谢谢你的回复！你的第一个建议有效，但当我尝试使用更简单的建议时，它通知我一个意外的“位置”。我还有一个小问题，我可以在使用主查询时使用子查询吗？这样我从数据库得到的只是医生的ID号？您缺少左JOIN的ON子句（
pa.doctorID=e.employeeID
）。@PaulSpiegel。非常感谢。