子查询加入同一张表

Question

所以我有一张员工表：

我正在尝试创建一个查询来显示该员工的所有信息，但我对 Employee_Reports_To 位有点困惑。我想要显示的是存储在此列中的 Employee_ID 的 First_Name 和 Last_Name（例如 Joe Bloggs ID 1，报告给员工 Joanne Blog ID 50）

这需要一个连接还是一个简单的选择。当我尝试时，数据库的链接有问题..

SELECT employee.*, departments.Department_Name, 
            jobTitle.Job_Title,
            (SELECT manager.First_Name, manager.Last_Name
                FROM Employee manager
                INNER JOIN employee AS employeeReportsTo
          ON manager.Employee_Reports_To = manager.Employee_ID   
            ) AS Reports_To
            FROM Employee employee
            LEFT JOIN Departments AS departments
                ON departments.Departments_ID = employee.Departments_ID
            LEFT JOIN Job_Title AS jobTitle
                ON jobTitle.Job_Title_ID = employee.Job_Title_ID
             ORDER BY `employee`.`Record_Active` DESC,
                     `employee`.`First_Name` ASC

任何帮助将不胜感激！

Answer 1

试试这个解决方案：

SELECT emp.*,
       manager.First_Name, 
       manager.Last_Name,
       departments.Department_Name, 
       jobTitle.Job_Title
FROM Employee emp
INNER JOIN Employee manager
ON emp.Employee_Reports_To = manager.Employee_ID
LEFT JOIN Departments AS departments
ON departments.Departments_ID = emp.Departments_ID
LEFT JOIN Job_Title AS jobTitle
ON jobTitle.Job_Title_ID = emp.Job_Title_ID
ORDER BY emp.Record_Active DESC,
             emp.First_Name ASC

使用Join 总是比使用Sub Query 更好。

已编辑：

由于Employee_Reports_To 列可以是null，因此最好使用LEFT JOIN 而不是INNER JOIN。像这样：

SELECT emp.*,
           manager.First_Name, 
           manager.Last_Name,
           departments.Department_Name, 
           jobTitle.Job_Title
FROM Employee emp
LEFT JOIN Employee manager                     --Changed Join here
ON emp.Employee_Reports_To = manager.Employee_ID
LEFT JOIN Departments AS departments
ON departments.Departments_ID = emp.Departments_ID
LEFT JOIN Job_Title AS jobTitle
ON jobTitle.Job_Title_ID = emp.Job_Title_ID
ORDER BY emp.Record_Active DESC,
         emp.First_Name ASC

Answer 2

查看您的代码，您可以使用两次 Employee（别名 e1 和 e2）一次用于员工，一次用于相关经理

SELECT e1.*
  , departments.Department_Name
  , jobTitle.Job_Title
  , e2.First_Name
  ,e2.Last_Name
  FROM Employee e1
  INNER JOIN Employee e2 ON e1.Employee_Reports_To = e2.Employee_ID
  LEFT JOIN Departments AS departments
      ON departments.Departments_ID = e1.Departments_ID
  LEFT JOIN Job_Title AS jobTitle
      ON jobTitle.Job_Title_ID = e1.Job_Title_ID
   ORDER BY e1.`Record_Active` DESC,
           e1.`First_Name` ASC