Sql加入返回Null结果

Question

我有4个表，我试图从中获取数据，但我的join语句返回null。我尝试了完整的，左右连接但没有成功。 Blow我提供了我的代码和图表，返回null值的表是ChannelData表。我试图从这张表中加总Vvalue：

DECLARE @bp varchar(4)
DECLARE @priority varchar(2)
DECLARE @startDate datetime
DECLARE @endDate datetime

SET @bp = 1710
SET @priority = 2

SET @endDate = (SELECT EndDate FROM BillingPeriod WHERE BillingPeriodClass_ID = 1 AND CODE = @BP)
SET @startDate = (SELECT EndDate FROM BillingPeriod WHERE BillingPeriodClass_ID = 1 AND CODE = @BP -1 )
SET @startDate = dateadd(minute, 1, @startDate)


SELECT  CGS.Description,
        CD.VValue, DI.Margin, @priority AS PRIORITY

FROM DataIntegrity AS DI

FULL JOIN CGS AS CGS ON CGS.ID = DI.CGS_ID_1 OR CGS.ID = DI.CGS_ID_2 
LEFT JOIN ChannelParameter AS CP ON CP.ID = CGS.ID
LEFT JOIN ChannelData AS CD ON CP.ID = CD.ID AND DI.CGS_ID_1 = CD.ID AND DI.CGS_ID_2 = CD.ID

where DI.Priority = @priority

 group by CGS.Description, CD.VValue,  DI.Margin

结果

db_diagram

另一个例子，我在尝试此查询时，在包含Report表而不是DI表时获取空值：

second result set

SELECT SUM(CD.VValue),  CGS.Description,
SUM(CD.VValue)

FROM CGS AS CGS

FULL JOIN ChannelParameter AS CP ON CP.ID = CGS.ID
FULL JOIN ChannelData AS CD ON CP.ID = CD.ID
FULL JOIN Report AS R on R.CGS_ID = CGS.ID

where -- CD.DDate BETWEEN @startDate AND @endDate 
             r.Description = 'rEGION EP'

 group by CGS.Description, CD.VValue'

Answer 1

我猜这个问题出现在加入中：

 LEFT JOIN ChannelData AS CD ON CP.ID = CD.ID AND DI.CGS_ID_1 = CD.ID AND DI.CGS_ID_2 = CD.ID

除此之外，您在CGS_ID_1 OR CGS_ID_2上使用了联接，在本节中，您使用 AND 进行此操作。这是对的吗？

CP.ID = CD.ID看起来也很可疑。它看起来像是从主键到主键的连接，而不是外键的主键。

Answer 2

有时，合适地加入表格不是解决方案。首先，通常建议甚至必须首先聚合并加入聚合，而不是先加入表并聚合。

然后，当我们只想检查表中的值是否存在时，我们通常希望将其作为WHERE子句中的条件，而不是连接中的条件。

有时我们希望将数据集与UNION ALL（或有时与UNION）合并。

第一个查询：每个CGS，边距和优先级的vvallue总和。好吧，CGS可以有多个边距和优先级，所以我们必须先找到它们。由于该表具有CGS_ID_1和CGS_ID_2，我们将使用UNION以获得每个CGS的边距和优先级。然后，vvalue与保证金和优先级无关;它只是每个CGS的总和，所以它将在结果中重复。

select cgs.description, di.margin, di.priority, coalesce(sum_vvalue, 0) as total
from cgs
join
(
  select cgs_id_1 as cgs_id, margin, priority from dataintegrity
  union
  select cgs_id_1 as cgs_id, margin, priority from dataintegrity
) di on di.cgs_id = cgs.id and di.priority = @priority
left join
(
  select id as cgs_id, sum(vvalue) as sum_vvalue
  from channeldata
  group by id
) cd on cd.cgs_id = cgs.id
order by cgs.description, di.margin, di.priority;

Secons查询：某个区域的每个CGS的vvallue总和。每个CGS可能有很多报告，我们必须在所需区域找到所有带有报告的CGS。这只是一次查询，应该使用IN或EXISTS。

select cgs.description, coalesce(sum_vvalue, 0) as total
from cgs
left join
(
  select id as cgs_id, sum(vvalue) as sum_vvalue
  from channeldata
  group by id
) cd on cd.cgs_id = di.cgs_id
where cgs.id in (select cgs_id from report where description = 'REGION EP')
order by cgs.description;