如何正确使用连接/子查询从多个表中选择数据？(PHP,MySQL的)

Question

我有三张桌子,如下图所示.

注意: projectheader表的Lead列存储员工ID.

我想要的是能够检索表格中的那个我的目标(Lead,显示该员工的主要名称)

我能够使用下面的查询来做到这一点.

SELECT DISTINCT
  projectdetails.ProjectDetailsID,
  projectheader.ProjectID,
  projectheader.ProjectName,
  projectheader.Lead,
  projectheader.StartDate,
  projectheader.EndDate,
  projectheader.Status,
  projectheader.Remarks,
  projectdetails.EmployeeID,
  employee.Firstname,
  employee.Lastname,
  Lead.Leadname
FROM
  projectheader,
  projectdetails,
  employee,
  ( SELECT
      projectheader.ProjectID AS projid,
      CONCAT(employee.Firstname,' ',employee.Lastname) AS Leadname
      FROM employee, projectheader, projectdetails 
      WHERE projectheader.ProjectID = projectdetails.ProjectID 
      AND projectheader.Lead = employee.EmployeeID
  ) AS Lead
WHERE projectheader.ProjectID = projectdetails.ProjectID
AND projectdetails.EmployeeID = employee.EmployeeID
AND projectheader.ProjectID = Lead.projid
AND projectdetails.ProjectID = Lead.projid

得到了这个结果:

我使用的查询很长,也许编写得不好,我想知道如何通过使用join或子​​查询使用更好的sql查询来实现相同结果的不同方式.(我在projectdetails.ProjectDetailsID的开头添加了一个DISTINCT,因为没有它会重复一些行).我正在寻找比我目前正在使用的查询更好的查询.

Answer 1

尝试这样的事情（没有测试过，你可以尝试一下）：

SELECT
  projectdetails.ProjectDetailsID,
  projectheader.ProjectID,
  projectheader.ProjectName,
  projectheader.Lead,
  projectheader.StartDate,
  projectheader.EndDate,
  projectheader.Status,
  projectheader.Remarks,
  projectdetails.EmployeeID,
  employee.Firstname,
  employee.Lastname,
  CONCAT(Lead.Firstname,' ',Lead.Lastname) AS Leadname
FROM
  projectheader,
  projectdetails,
  employee,
  employee as Lead
WHERE projectheader.ProjectID = projectdetails.ProjectID
AND projectdetails.EmployeeID = employee.EmployeeID
AND projectheader.Lead = Lead.EmployeeID