SQL笛卡尔连接问题

Question

我有三张桌子

• A：A.pID 主键，A.Name nvarchar(250)
• B：B.pID 主键，B.Name nvarchar(250)
• C: C.pID 主键，C.Name nvarchar(250)

A 和 B 之间存在 m 到 n 关系（表 lA_B 主键为 lA_B.pID 和 .pInstanceA 表 A 的外键和 .pInstanceB 表 B 的外键）

A 和 C 之间存在 m 对 n 关系（table lA_C 主键 lA_C.pID 和 .pInstanceA 表 A 的外键和 .pInstanceB 表 C 的外键）

• A1 与 B1、B2 和 C1 相关
• A2 与 B3 和 C2、C3 相关
• A3 与 B4 相关
• A4 与 C4 相关
• A5 没有关系

这是我的 SQL：

CREATE TABLE [dbo].[A]( [pID] [bigint] NOT NULL, [Name] [nvarchar](250) NULL )
CREATE TABLE [dbo].[B]( [pID] [bigint] NOT NULL, [Name] [nvarchar](250) NULL )
CREATE TABLE [dbo].[C]( [pID] [bigint] NOT NULL, [Name] [nvarchar](250) NULL)
CREATE TABLE [dbo].[lA_B]( [pID] [bigint] NOT NULL, [pInstanceA] [bigint] NULL, [pInstanceB] [bigint] NULL )
CREATE TABLE [dbo].[lA_C]( [pID] [bigint] NOT NULL, [pInstanceA] [bigint] NULL, [pInstanceB] [bigint] NULL )

INSERT INTO [dbo].[A] ([pID] ,[Name]) VALUES (1,'A1')
INSERT INTO [dbo].[A] ([pID] ,[Name]) VALUES (2,'A2')
INSERT INTO [dbo].[A] ([pID] ,[Name]) VALUES (3,'A3')
INSERT INTO [dbo].[A] ([pID] ,[Name]) VALUES (4,'A4')
INSERT INTO [dbo].[A] ([pID] ,[Name]) VALUES (5,'A5')

INSERT INTO [dbo].[B] ([pID] ,[Name]) VALUES (1,'B1')
INSERT INTO [dbo].[B] ([pID] ,[Name]) VALUES (2,'B2')
INSERT INTO [dbo].[B] ([pID] ,[Name]) VALUES (3,'B3')
INSERT INTO [dbo].[B] ([pID] ,[Name]) VALUES (4,'B4')

INSERT INTO [dbo].[C] ([pID] ,[Name]) VALUES (1,'C1')
INSERT INTO [dbo].[C] ([pID] ,[Name]) VALUES (2,'C2')
INSERT INTO [dbo].[C] ([pID] ,[Name]) VALUES (3,'C3')
INSERT INTO [dbo].[C] ([pID] ,[Name]) VALUES (4,'C4')

INSERT INTO [dbo].[lA_B] ([pID],[pInstanceA],[pInstanceB])   VALUES   (1,1,1)
INSERT INTO [dbo].[lA_B] ([pID],[pInstanceA],[pInstanceB])   VALUES   (2,1,2)
INSERT INTO [dbo].[lA_B] ([pID],[pInstanceA],[pInstanceB])   VALUES   (3,2,3)
INSERT INTO [dbo].[lA_B] ([pID],[pInstanceA],[pInstanceB])   VALUES   (4,3,4)

INSERT INTO [dbo].[lA_C] ([pID],[pInstanceA],[pInstanceB])   VALUES   (1,1,1)
INSERT INTO [dbo].[lA_C] ([pID],[pInstanceA],[pInstanceB])   VALUES   (2,2,2)
INSERT INTO [dbo].[lA_C] ([pID],[pInstanceA],[pInstanceB])   VALUES   (3,2,3)
INSERT INTO [dbo].[lA_C] ([pID],[pInstanceA],[pInstanceB])   VALUES   (4,4,4)

这个查询：

SELECT
  A.Name AS A, 
  B.Name AS B, 
  C.Name AS C
FROM
  A  
  left JOIN lA_B  ON (A.pID = lA_B.pInstanceA)
  left JOIN B     ON (B.pID = lA_B.pInstanceB)
  left JOIN lA_C  ON (A.pID = lA_C.pInstanceA)
  left JOIN C     ON (C.pID = lA_C.pInstanceB) 

返回

A1 B1 C1 A1 B2 C1 A2 B3 C2 A2 B3 C3 A3 B4 空 A4 空 C4 A5 空 空

现在的问题:-) 如何查询接收

A1 B1 空 A1 B2 空 A1 空 C1 A2 B3 空 A2 空 C2 A2 空 C3 A3 B4 空 A4 空 C4 A5 空 空

问题是当我用 B 和 C 进行连接时，结果是 B C 的所有组合。我怎样才能消除这种情况？

Answer 1

你也许可以用 UNION 做到这一点：

SELECT A.Name AS A, B.Name AS B, NULL AS C 
FROM A
left JOIN lA_B ON (A.pID=lA_B.pInstanceA) 
left JOIN B ON (lA_B.pInstanceB=B.pID) 
UNION
SELECT A.Name AS A, NULL AS B, C.Name AS C 
FROM A
left JOIN lA_C ON (A.pID=lA_C.pInstanceA) 
left JOIN C ON (lA_C.pInstanceB=C.pID)

第一部分选择A和B的所有组合，第二部分选择A和C的所有组合。

如果你想过滤掉像 (A4,NULL,NULL) 这样的行，因为已经有一行 (A4,NULL,C4)，试试这个查询：

SELECT A.Name AS A, B.Name AS B, NULL AS C 
FROM A
LEFT JOIN lA_B ON (A.pID=lA_B.pInstanceA) 
LEFT JOIN B ON (lA_B.pInstanceB=B.pID) 
WHERE b.name is not null 
    or not exists(select * from lA_C where A.pID=lA_C.pInstanceA) 
UNION
SELECT A.Name AS A, NULL AS B, C.Name AS C 
FROM A
LEFT JOIN lA_C ON (A.pID=lA_C.pInstanceA) 
LEFT JOIN C ON (lA_C.pInstanceB=C.pID)
WHERE c.name is not null 
ORDER BY A,B,C

对于 B 上的连接，这表示包括在 B 中匹配或在 C 中没有匹配的行。在 C 上的连接仅包括在 C 中匹配的行。不匹配的行将从 B 上的连接中获取。

请注意，UNION 会过滤掉重复的行，例如 DISTINCT。要包含每一行，您可以使用 UNION ALL。

Answer 2

我认为这样做：

select a,
       case when b='zzz' then null else b end as b,
       case when c='zzz' then null else c end as c
from (SELECT  A.Name AS A
             ,b.Name as b
             ,'zzz' as c
      FROM  A    
      JOIN lA_B  ON (A.pID = lA_B.pInstanceA)  
      JOIN B     ON (B.pID = lA_B.pInstanceB)  
      union
      select  a.Name
             ,'zzz'
            ,c.NAme
      from A
      left JOIN lA_C  ON (A.pID = lA_C.pInstanceA)  
      left JOIN C     ON (C.pID = lA_C.pInstanceB)) as a