Php 在MYSQL查询和左连接上显示空值

我有一个疑问：

SELECT 
    pupils.id_pupil
  , name
  , surname
  , (select round(avg(qualifications.calificacion),2) from qualifications where id_pupil = pupils.id_pupil and qualifications.id_trimester=incidents.id_trimester and type_qualification='class') as media
  , (select count(calificacion) from qualifications where id_pupil = pupils.id_pupil and qualifications.id_trimester=incidents.id_trimester and type_qualification='class') as count
  , COALESCE(sum(type_incident='miss' and level=1 and class='A' and id_trimester=1),0) as misses
  , COALESCE(sum(type_incident='delay' and level=1 and class='A' and id_trimester=1),0) as delays
  , COALESCE(sum(type_incident='attitude' and level=1 and class='A' and id_trimester=1),0) as attitude
  , COALESCE(sum(type_incident='miss_justif' and level=1 and class='A' and id_trimester=1),0) as misses_justificadas 
FROM 
    pupils 
    left join incidents         ON incidents.id_pupil=pupils.id_pupil
WHERE 
    level=1
    and class='A' 
    and id_trimester=1 
GROUP BY id_pupil

表：

pupils: (id_pupil, name, surname, email, user, pass, level, class)
incidents: (id_incidents, date, time, type_incident, comments, id_pupil, id_user, subject, id_trimester)
qualifications: (id_qualification, qualification, date, time, subject, id_pupil, id_user, id_trimester, type_qualification)

它可以工作，但不显示四个值为0的未命中、延迟、姿态和未命中的行。那一行被省略了

如何显示所有行

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谢谢

您的查询有一些问题。我做了一个修改，也许你能给你想要的结果。也许你应该把你的名字、姓氏、媒体和计算值放在你的群组中，但这取决于你的需求

SELECT 
 pupils.id_pupil
 , name
 , surname
 , IFNULL(qua.media, 0) media
 , IFNULL(qua.count, 0) count
 , sum( case when type_incident='miss' then 1 else 0 end ) as misses
 , sum( case when type_incident='delay' then 1 else 0 end ) as delays
 , sum( case when type_incident='attitude' then 1 else 0 end ) as attitude
 , sum( case when type_incident='miss_justif' then 1 else 0 end ) 
 as misses_justificadas 
FROM 
 pupils 
 LEFT JOIN incidents ON incidents.id_pupil=pupils.id_pupil
 LEFT JOIN
 (select id_pupil, id_trimester, 
 count(qualification) count, 
 round(avg(qualifications.qualification),2) media
 from qualifications 
 where type_qualification='class'
 group by id_pupil, id_trimester ) as qua
 ON ( qua.id_pupil = pupils.id_pupil )
WHERE 
 level=1
 and class='A' 
 and qua.id_trimester=1 
GROUP BY pupils.id_pupil

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你的算术全搞糟了。您在where语句中的条件与SUM语句中的条件完全相同。您可以发布事件表吗？@RubenGiaquinto我已经发布了该表。在这种情况下，您没有得到任何记录的原因是，您的where子句检查id_trimester，该id_trimester位于连接事件表的左侧。如果没有事件，这个字段有一个空值，所以实际上您已经通过检查它是否为1完成了一个内部连接。您需要在子查询中检查它。但是我建议使用一些类似于@JorgeCampos下面给出的代码，这样就消除了select子句中的子查询。我猜你是西班牙人，我也是，如果你不介意我用西班牙语回答的话。El id_trimester=1 me falla ya que dice que es ambiguo。在基多，我的职能没有改变结果，这是一个0。我是萨尔塔岛上的一员，没有什么特别之处：S@AlonsoGonzálezNestal-如果qua.media为null，则可以使用IFNULL（qua.media，0）获得0的值。此时，在sqlfiddle上进行数据采样会很好。我认为这是因为事件中缺少值（因为它是一个左连接），而这

qua.id\u trimester=incents.id\u trimester

导致了这样的问题。也许你会像最初发布的那样使用subselect返回到旧的select，也许它会起作用。但是为了更好地帮助你，最好有sqlfiddle。别担心@JorgeCampos完成了95%的工作，值得表扬。