合并来自不同表的 2 SQL 个查询

Question

我有一个包含医生和过去预约的预约数据库。当用户想预约医生时，他会选择一个住院医师，之后他将根据该住院医师显示医生。现在是棘手的部分：

医生将按照过去预约的顺序显示，这意味着如果我过去去看了两位心脏病专家，我预约的最后一位将首先显示，在他之后是另一位，在他们之后，我从未预约过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约会并按日期降序排列，然后去找所有医生并将其他医生添加到列表底部。

我正在尝试这个查询：

(
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运行 union 命令上面的第一个查询，没有括号我得到了正确的顺序，一旦括号查询结果变为错误的.

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做到这一点而不丢失我需要的降序。

Answer 1

如果您想要特定顺序的结果，则需要在最外层查询中指定 order by。

在MySQL中，这是在之后的union。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returns两栏。如果你想要一列，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;

我还应该注意第一个查询中 group by 的使用。这是获得正确的最后预约时间所必需的。

而且，如果我假设所有医生都是雇员，这可以大大简化：

 select e.employeeID
 from employees e left join
      (select pa.doctorID, max(apptime) as last_apptime
       from pastappointments pa
       where pa.insuredID = 1 and pa.residency = 'cardio'
       group by pa.doctorId
      ) pa
      on e.EmployeeId = pa.DoctorId
 where e.Residency = 'cardio' -- or pa.doctorId is not null
 order by (last_apptime is not null) desc, last_apptime;

这也假定 e.Residency 与 pa.residency 相同。如果不是这种情况，则需要 or 条件。

这样写查询比较合理（假设所有医生都是员工）。

Answer 2

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

Answer 3

您可以使用 LEFT JOIN 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

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