SQL外部连接？

Question

我喜欢这些表：

Fields (ID int, Name varchar(20)) 

ID  Name 
=============== 
1  FieldName 
2  FeildDesc 

Container (ID int, Name varchar(100)) 

ID  Name 
==================================== 
1  C1  
2  C2 
3  C3 

ContainerField (ContainerId int, FieldId int, FieldValue varchar(100)) 

ContainerId FieldId FieldValue 
==================================== 
1    1   Container1 
1    2   Container1 Desc 
2    1   Container2 
2    2   Container3 Desc 
3    1   Container3 

我希望有我的结果是这样的：

ContainerId Name   Desc 
=================================== 
1    Container1 Container1 Desc 
2    Container2 Container2 Desc 
3    Container3 NULL 

我试图做LEFT OUTER ContainerField和领域，而且没”之间的连接似乎没有工作。如果有人能帮忙，我会很感激。

Answer 1

加入表格两次，一次为名称，一次为描述。

select 
    c.Id as ContainerId, 
    cfn.FieldValue as Name, 
    cfd.FieldValue as Desc 
from 
    Container c 
    left join ContainerField cfn 
    on cfn.ContainerId = c.ID and cfn.FieldId = 1 
    left join ContainerField cfd 
    on cfd.ContainerId = c.ID and cfd.FieldId = 2 

正如您所看到的，不是组或聚合。另外，查询中并不需要Fields tabel本身，不过在数据库中使用Fields tabel本身是很好的，因为它会强制引用完整性（如果您拥有正确的密钥）并提供一种文档。但你也可以使用枚举。

或者，如果您不只有两个字段，但是它们的数量未知，您可能需要考虑pivot tables。我必须承认，对于这些人我并不是很擅长，特别是在TSQL中，所以我会为您提供一个链接供您探索：http://technet.microsoft.com/en-us/library/ms177410(v=sql.105).aspx或者希望别人能给你一个具体的例子。

Answer 2

你做的似乎只是需要一个left join：

select c.id, c.name, cf.FieldValue 
from Container c join 
    ContainerField cf 
    on cf.ContainerId = c.id and cf.FieldId = 2; 

当然，在你的问题中的数据是有点误导。我假设name列意在来自Container表。

Answer 3

不需要外连接，内连接就可以做到。您也没有使用Container中的任何内容（如果您想显示容器名称而不是ID，则可以使用它进行内部连接以获取该内容）。

SELECT c.id AS ContainerID, 
     MAX(CASE WHEN f.name = 'FieldName' THEN c.FieldValue END) AS Name, 
     MAX(CASE WHEN f.name = 'FieldDesc' THEN c.FieldValue END) AS `Desc` 
FROM ContainerField AS c 
JOIN Fields AS f ON c.FieldID = f.id 
GROUP BY ContainerID 

DEMO

请注意，您在Fields表有一个拼写错误：FeildsDesc应该是FieldsDesc。

Answer 4

我觉得这是你所追求的：

SELECT t1.ContainerId, t1.FieldValue, t2.FieldValue 
FROM ContainerField t1 
LEFT JOIN ContainerField t2 ON t1.ContainerId = t2.ContainerId 
     AND t1.FieldId = 1 and t2.FieldId = 2 

EDIT（SQL Fiddle）：

SELECT DISTINCT t1.ContainerId, 
(
    SELECT t2.FieldValue FROM ContainerField t2 
    WHERE t2.FieldId = 1 AND t1.ContainerId = t2.ContainerId 

) AS [Name], 
(
    SELECT t3.FieldValue FROM ContainerField t3 
    WHERE t3.FieldId = 2 AND t1.ContainerId = t3.ContainerId 
) AS [Desc] 
FROM ContainerField t1 

Answer 5

可能会在我的两分钱，以及抛出。这会让你得到你所需要的。这是在Oracle中完成的，但应该也一样。

编辑：SQL Fiddle Demo作为一个工作的例子。

Select * From (

WITH 
/* Example Tables.*/ 
Container As 
(
    Select 1 as ID, 'C1' as Name From Dual Union All  
    Select 2 as ID, 'C2' as Name From Dual Union All 
    Select 3 as ID, 'C3' as Name From Dual 
) 

,ContainerField as 
(
    Select 1 as ContainerId, 1 as FieldId, 'Container1  ' as FieldValue From Dual UNION ALL 
    Select 1 as ContainerId, 2 as FieldId, 'Container1 Desc' as FieldValue From Dual UNION ALL 
    Select 2 as ContainerId, 1 as FieldId, 'Container2  ' as FieldValue From Dual UNION ALL 
    Select 2 as ContainerId, 2 as FieldId, 'Container2 Desc' as FieldValue From Dual UNION ALL 
    Select 3 as ContainerId, 1 as FieldId, 'Container3  ' as FieldValue From Dual 
) 

/*Code*/ 
Select 
    C.ID, 
    Max(Case When CF.FieldId = 1 Then FieldValue End) as Name, 
    Max(Case When CF.FieldId = 2 Then FieldValue End) as Descrption 
From 
    Container C 
    Left Join ContainerField CF on C.ID = CF.ContainerID 

Group By 
    C.ID, 
    C.Name 
Order By 
    C.ID 
) 

输出：

CONTAINERID  NAME  DESCRPTION 
1    Container1 Container1 Desc 
2    Container2 Container2 Desc 
3    Container3