包括在树表上的 INNER JOIN SQL Server 查询中具有空值的记录

Question

我正在查询一个对自身具有嵌套引用的表，并且我需要获取对我正在使用的 INNER JOIN 值具有空引用的记录。换句话说，树表记录可能不会达到相同的深度，我需要全部获取。

我使用 atm 的查询是：

SELECT DISTINCT <valuesThatINeed>
FROM atable foo
   INNER JOIN treetable bar ON foo.id_bar = bar.id
   INNER JOIN treetable bar2 ON bar.id_bar = bar2.id
   INNER JOIN treetable bar3 ON bar2.id_bar = bar3.id
WHERE
   <constraints>

我读到我可以通过添加一个 OR 语句来扩展 INNER JOIN 条件，询问 bar.id_bar 是否为 NULL，但如果这样做，我将无法达到更高的级别：

SELECT DISTINCT <valuesThatINeed>
FROM atable foo
   INNER JOIN treetable bar ON foo.id_bar = bar.id 
   INNER JOIN treetable bar2 ON bar.id_bar = bar2.id OR (bar.id_bar IS NULL) 
   INNER JOIN treetable bar3 ON bar2.id_bar = bar3.id
WHERE
   <constraints>

Answer 1

将INNER JOIN 更改为LEFT JOIN：

SELECT DISTINCT <valuesThatINeed>
, CASE 
     WHEN barN.id IS NULL 
         THEN 'Your previous level with BarID = ' 
               + CAST(barN-1.id AS NVARCHAR(MAX)) 
               + ' is the deepest level'
      ELSE ''
  END
FROM atable foo
   LEFT JOIN treetable bar ON foo.id_bar = bar.id 
   LEFT JOIN treetable bar2 ON bar.id_bar = bar2.id OR (bar.id_bar IS NULL) 
   LEFT JOIN treetable bar3 ON bar2.id_bar = bar3.id
   ...
   LEFT JOIN treetable barN ON barN-1.id_bar = barN.id
WHERE
   <constraints>

您可以继续进行LEFT 连接，直到达到您的barN.id IS NULL 的深度，这意味着您已达到N-1 的最深层次。

但是，如果您要创建层次结构，那么这不是可扩展的方式。在网络上搜索递归 CTE (here is a possible hint which you can try to adapt to your situation)。

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