SQL - 1 个父表，2 个子表 - 为子表中的每一行返回单行

Question

表A

• 家长ID
• 姓名

表B

• B key
• 家长ID
• 描述

表C

• CKey
• 家长ID
• 描述

我需要为 B/A 中与父 id 匹配的每个组合数据行返回 1 行，如果其中一个子表的行数多于另一个，则应返回该描述的一行，其中包含空值。

例如，如果数据如下

表A

1   FirstParent
2   Second Parent

表B

1   1   BDesc1
2   1   BDesc2
3   2   P2BDesc1

表C

1   1   CDesc1
2   2   P2CDesc1
3   2   P2CDesc2

如果我基于FirstParent检索，结果应该是:

1   FirstParent   BDesc1   CDesc1
1   FirstParent   BDesc2   NULL

如果我基于SecondParent检索，结果应该是:

2   SecondParent   P2BDesc1   P2CDesc1
2   SecondParent   NULL       P2CDesc2

有没有办法不需要工会就可以做到这一点？

Answer 1

declare @ParentID int
set @ParentID = 1

select a.name,
       bc.descb,
       bc.descc
from   TableA as a
  cross join (select b.descb,
                     c.descc
              from   (select *,
                             row_number() over(order by b.bkey) as rn
                      from   TableB as b
                      where  b.parentid = @parentid) as b
                full outer join 
                     (select *,
                             row_number() over(order by c.ckey) as rn
                      from   TableC as c
                      where  c.parentid = @parentid) as c
                  on b.rn = c.rn) as bc
where  a.parentid = @parentid  

在这里试试:https://data.stackexchange.com/stackoverflow/qt/112538/

编辑:使用ExternalKey查询多个ParentID的版本

建议索引:

create index IX_B_ParentID on TableB(ParentID) include (DescB)
create index IX_C_ParentID on TableC(ParentID) include (DescC)

我将创建一个表变量来保存与ExternalKey 匹配的ParentID，然后在查询中使用它而不是TableA。

declare @ExternalKey int = 1

declare @T table(ParentID int primary key, Name varchar(20))
insert into @T (ParentID, Name)
select ParentID, NAme
from TableA
where ExternalKey = @ExternalKey


select a.name,
       bc.descb,
       bc.descc
from   @T as a
  inner join (select b.descb,
                     c.descc,
                     coalesce(b.ParentID, c.ParentID) as ParentID
              from   (select b.ParentID,
                             b.DescB,
                             row_number() over(partition by b.ParentID order by b.bkey) as rn
                      from   TableB as b
                      where  b.parentid in (select ParentID from @T)) as b
                full outer join
                     (select c.ParentID,
                             c.DescC,
                             row_number() over(partition by c.ParentID order by c.ckey) as rn
                      from   TableC as c
                      where  c.parentid in (select ParentID from @T)) as c
                  on b.rn = c.rn and
                     b.ParentID = c.ParentID) as bc
    on a.ParentID = bc.ParentID