Question

我有3个表之间的关系，我们称之为tree，parent和children。

我想（总是）获取树，如果没有删除则获取父级，或者如果未删除，则删除未删除的子级和子级。

以下查询使用异常，它不会获取树。

SELECT * 
FROM tree t
LEFT JOIN parent p ON t.id = p.tree_id
LEFT JOIN children c ON p.id = c.parent_id
WHERE t.id = 2599 
    AND (p.deleted = false OR (p.deleted = true AND pc.deleted = false));

有什么想法吗？

Answer 1

正如评论中所提到的，看起来你在第二次加入时遇到了问题。

此外，您的WHERE子句会将LEFT JOIN转换为INNER JOIN s：
- NULL永远不会等于TRUE或FALSE

我会尝试这样的事情......

SELECT
  * 
FROM
  tree       AS t
LEFT JOIN
  (
    parent     AS p
  LEFT JOIN
    children   AS c
      ON  c.parent_id = p.id
      AND c.deleted   = false
  )
    ON  (t.id = p.tree_id)
    AND (c.id IS NOT NULL OR p.deleted = false)
WHERE
  t.id = 2599

children仅与parent相关联，因为它们未被删除。

如果tree未被删除，或者有子女（我们已经确保只有在未删除时才包含这些内容），该加入才会与parent相关联

编辑：

我从不喜欢使用RIGHT JOIN，但也许它可以更具可读性？我不这么认为，但我会把它包括在内以便完整。

SELECT
  * 
FROM
  parent     AS p
LEFT JOIN
  children   AS c
    ON  c.parent_id = p.id
    AND c.deleted   = false
RIGHT JOIN
  tree       AS t
    ON  (t.id = p.tree_id)
    AND (c.id IS NOT NULL OR p.deleted = false)
WHERE
  t.id = 2599

Answer 2

实际上并不困难。（编辑：我希望我从来没有说过这个;-)只确保正确处理外连接记录（即检查为空）。

SELECT * 
FROM tree t
LEFT JOIN parent p ON t.id = p.tree_id
LEFT JOIN children c ON p.id = c.parent_id and c.deleted = false 
WHERE t.id = 2599 
AND (p.id is null OR p.deleted = false OR c.deleted = false) -- parent okay?
AND (c.id is null OR c.deleted = false) -- child okay?

获取过滤后的树状关系

2 个答案: