Question

所以我有两个表格结构如下：

CREATE TABLE #nodes(node int NOT NULL);
ALTER TABLE #nodes ADD CONSTRAINT PK_nodes PRIMARY KEY CLUSTERED (node);

CREATE TABLE #arcs(child_node int NOT NULL, parent_node int NOT NULL);
ALTER TABLE #arcs ADD CONSTRAINT PK_arcs PRIMARY KEY CLUSTERED (child_node, parent_node);

INSERT INTO #nodes(node)
VALUES (1), (2), (3), (4), (5), (6), (7);

INSERT INTO #arcs(child_node, parent_node)
VALUES (2, 3), (3, 4), (2, 6), (6, 7);

如果我有两个节点，请说1和2.我想要一个根节点列表。在这种情况下，它将是1,4和7.如何编写查询以获取该信息？

我写了一篇文章，但遇到了一个问题，我不能在CTE的递归部分使用LEFT连接，原因不明。如果我被允许进行LEFT JOIN，那么这个查询将起作用。

WITH root_nodes
AS (
    -- Grab all the leaf nodes I care about and their parent
    SELECT n.node as child_node, a.parent_node
    FROM #nodes n
    LEFT JOIN #arcs a
      ON n.node = a.child_node
    WHERE n.node IN (1, 2)

    UNION ALL

    -- Grab all the parent nodes
    SELECT rn.parent_node as child_node, a.parent_node
    FROM root_nodes rn
    LEFT JOIN #arcs a -- <-- LEFT JOINS are Illegal for some reason :(
      ON rn.parent_node = a.child_node
    WHERE rn.parent_node IS NOT NULL
)
SELECT DISTINCT rn.child_node as root_node
FROM root_nodes rn
WHERE rn.parent_node IS NULL

有没有办法可以重构查询以获得我想要的内容？我无法对数据进行重组，我宁愿远离临时表或不必做任何昂贵的事情。

谢谢，劳尔

Answer 1

如何将LEFT JOIN移出CTE？

WITH root_nodes
AS (
    -- Grab all the leaf nodes I care about
    SELECT NULL as child_node, n.node as parent_node
    FROM #nodes n
    WHERE n.node IN (1, 2)

    UNION ALL

    -- Grab all the parent nodes
    SELECT rn.parent_node as child_node, a.parent_node
    FROM root_nodes rn
        JOIN #arcs a
      ON rn.parent_node = a.child_node
)
SELECT DISTINCT rn.parent_node AS root_node
FROM root_nodes rn
    LEFT JOIN #arcs a
  ON rn.parent_node = a.child_node
WHERE a.parent_node IS NULL

结果集为1,4,7。

T-SQL在层次结构中获取根节点

1 个答案: