Question

我正在加入两个不同的关系计划,如:

TableA1    ->    TableA2      ->  TableB     ->  TableC    : <<< RELATIONSHIP A
enter code here

产出必须照样:

   A1columnId   A2columnId  BcolumnId   CcolumnId
   1            1           1           1
   2            null        2           2
   3            null        3           3
   4            2           4           4
   5            null        5           5

如果存在这两个关系A &B,我需要看到A2colId在以下关系中无效:

And the 2nd relationship scheme looks like so: TableA1->TableB-TableC <<<< RELATIONSHIP B (This scheme has no clue about TableA2)

您将如何加入A2Column公司,以取得结果。同上。

Answer 1

<代码>LEFT JOIN, 而不是定期加入,即使右桌上没有数据,填写在<代码>null上。因此,当你加入A1和A2时,你会这样做:

SELECT * 
  FROM A1 
       LEFT JOIN A2 
          ON A1.idlink = A2.idlink 
       JOIN B 
          ON A1.id = B.id 
       JOIN C 
          ON A1.id = C.id;

Answer 2

SELECT *
FROM TableA A1
  LEFT JOIN TableA A2 ON A1.ColumnId1 = A2.ColumnId2
  INNER JOIN TableB B ON B.ColumnId IN (A1.ColumnId1, A2.ColumnId2)
  INNER JOIN TableC C ON B.ColumnId = C.ColumnId

if I got you right.

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