根据条件从多个表中获取字段名

Question

我有以下 mysql 查询：

SELECT 
  u.ID,
  u.StudentCode,
  c.classname,
  s.STUDENTNAME,
  u.UserCode,
  g.ForeName AS MessageSentBy,
  u.Message,
  u.ForTchrSname AS MessageSentTo,
  DATE_FORMAT(u.datetime, '%d-%m-%y') AS datesent,
  u.RecordStatus 
FROM
  tblupdates u,
  tblguardian g,
  tblstudent s,
  tblclass c 
WHERE u.UserCode = g.GuardianCode 
  AND u.StudentCode = s.STUDENTCODE 
  AND (
    YEAR(u.DateTime) = '2013' 
    AND MONTH(u.DateTime) = '09'
  ) 
  AND u.RecordStatus = 'N' 
  AND c.classcode = s.classcode 
ORDER BY u.datetime DESC 

此查询的结果如下所示：

现在在 2013 年和 09 月的 tblupdates 中，我有以下行：

现在在上面的结果中，我正在获取id=138 的数据，但我想显示所有数据。不显示其他行的原因是(usercode-30000,admin) 这些发生在tblteacher 而不是tblguardian。所以现在我希望如果usercode 来自tblguardian 它显示监护人姓名，但如果它来自tblteacher 它显示teachername 但显示所有行。Tblteacher 和 tblupdates 可以分别由教师代码和用户代码加入。

我尝试了以下查询，但它没有返回任何行：

SELECT 
  u.ID,
  u.StudentCode,
  c.classname,
  s.STUDENTNAME,
  u.UserCode,
  IF(
    u.usercode NOT IN (g.guardiancode),
    t.teachername,
    g.ForeName
  ) AS MessageSentBy,
  u.Message,
  u.ForTchrSname AS MessageSentTo,
  DATE_FORMAT(u.datetime, '%d-%m-%y') AS datesent,
  u.RecordStatus 
FROM
  tblupdates u,
  tblguardian g,
  tblstudent s,
  tblclass c,
  tblteacher t 
WHERE u.UserCode = g.GuardianCode 
  AND u.StudentCode = s.STUDENTCODE 
  AND t.teachercode = u.usercode 
  AND (
    YEAR(u.DateTime) = '2013' 
    AND MONTH(u.DateTime) = '09'
  ) 
  AND u.RecordStatus = 'N' 
  AND c.classcode = s.classcode 
ORDER BY u.datetime DESC 

请帮帮我

Answer 1

使用CASE 尝试此操作，我已使用LEFT 加入您的表格，因此可能会有重复我使用GROUP BY 也使用u.id 但对正确的列名有点困惑请把正确的列放入分组

SELECT 
  u.ID,
  u.StudentCode,
  c.classname,
  s.STUDENTNAME,
  u.UserCode,  
  (CASE 
  WHEN u.usercode = t.teachercode THEN t.teachername 
  WHEN u.usercode = g.guardiancode THEN g.ForeName 
  ELSE END ) AS MessageSentBy,
  u.Message,
  u.ForTchrSname AS MessageSentTo,
  DATE_FORMAT(u.datetime, '%d-%m-%y') AS datesent,
  u.RecordStatus 
FROM
  tblupdates u
  LEFT JOIN  tblguardian g ON (u.UserCode = g.GuardianCode )
  LEFT JOIN  tblstudent s ON (u.StudentCode = s.STUDENTCODE )
  LEFT JOIN  tblclass c ON (c.classcode = s.classcode )
  LEFT JOIN   tblteacher t  ON (t.teachercode = u.usercode)
WHERE   u.RecordStatus = 'N' 
 AND  (
    YEAR(u.DateTime) = '2013' 
    AND MONTH(u.DateTime) = '09'
  )  
GROUP BY u.id
ORDER BY u.datetime DESC