How to select data from multiple tables using joins/subquery properly? (PHP-MySQL)

问题

I have three tables as shown in below image.

Note: Lead column of projectheader table stores an employee id.

What I want to have is be able to retrieve something like the one in table my goal(Lead, displays the lead name of that employee)

I was able to do that using the query below.

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

And got this result:

The query that I used is quite long and perhaps not well written, I want to know a different way on how I could achieve the same result using a better sql query either by using join or a subquery. (I added a DISTINCT on the beginning of the projectdetails.ProjectDetailsID because without it some rows are duplicated). I'm in search for a better query than the one I'm currently using.

回答1:

Try something like this (haven't tested it, you can give it a try):

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

回答2:

Try this Query i hope its work for you

SELECT pd.ProjectDetailsID,ph.*,e.* FROM
`projectdetail` pd
INNER JOIN projectheader ph ON ph.ProjectID = pd.ProjectID 
INNER JOIN employee e ON e.EmployeeID = pd.EmployeeID

来源：https://stackoverflow.com/questions/11175901/how-to-select-data-from-multiple-tables-using-joins-subquery-properly-php-mysq

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php

mysql

join

subquery