SQL select 查询中的条件变量

Question

我正在尝试编写一个使用 Nested Interval Hierarchy 数据库模型的 SQL 查询。

--Given a parent Id, this query retrieves the position of the youngest child which can be inserted into the table
SELECT TOP 1
    --compute values based on the youngest "sibling id" (the one with the highest value)
    (parent.m_11 * (FLOOR(child.m_11/child.m_12)+2) - parent.m_12) as m_11,
    (parent.m_11) as m_12, 
    (parent.m_21 * (FLOOR(child.m_11/child.m_12)+2) - parent.m_22) as m_21,
    (parent.m_21) as m_22
    FROM my_so_table child
        --grabs all children of the parent
        JOIN my_so_table parent
          ON parent.Id = 1
         AND parent.m_21 = child.m_22
         AND parent.m_11 = child.m_12
    --the operation "Floor(child.m_11 / child.m_12) is the siblingId that I need to do math on above
    ORDER By FLOOR(child.m_11/child.m_12) DESC
GO

有架构：

GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[my_so_table](
    [m_11] [int] NOT NULL,
    [m_12] [int] NOT NULL,
    [m_21] [int] NOT NULL,
    [m_22] [int] NOT NULL,
    [Id] [int] IDENTITY(1,1) NOT NULL)
GO

INSERT INTO [dbo].[my_so_table] VALUES (2,1,1,0); --1.
INSERT INTO [dbo].[my_so_table] VALUES (3,1,1,0); --2.
INSERT INTO [dbo].[my_so_table] VALUES (3,2,2,1); --1.1
INSERT INTO [dbo].[my_so_table] VALUES (4,1,1,0); --3.

使用上述架构，运行上述查询 parentId 为 1 正确 returns

5,2,3,1

正确表示了要插入到Id为1的parent下的新节点的矩阵。运行上面的查询parentId为2 returns 一个空列表，当它应该返回时

5,3,2,1

表示Id为2的parent下第一个child的矩阵

这是错误的，因为我的查询没有处理 parent 中没有 children 的情况。如果没有parent的children，Floor(child.m_11 / child.m_12)的结果应该是-1。我如何更改我的查询以完成此操作？

Answer 1

我找到了我的解决方案。我一直在回避 greatest-n-by-group，而实际上我不明白 group-by 是如何工作的。

--Given a parent Id, this query retrieves the position of the youngest child which can be inserted into the table
--insert into my_so_table(m_11, m_12, m_21, m_22)
SELECT TOP 1
    --compute values based on the youngest 'sibling id' (the one with the highest value)
    (parent.m_11 * (ISNULL(siblingId, 0) + 2) - parent.m_12) as m_11,
    (parent.m_11) as m_12, 
    (parent.m_21 * (ISNULL(siblingId, 0) + 2) - parent.m_22) as m_21,
    (parent.m_21) as m_22
    FROM my_so_table parent
        --grabs all children of the parent
        LEFT JOIN (
            --Grabs the youngest sibling for each sibling chain
            SELECT
                child.m_12,
                child.m_22,
                Max(Floor(child.m_11 / child.m_12)) as siblingId
            FROM my_so_table child
            Group By child.m_12, child.m_22
        ) child
        on(parent.m_21 = child.m_22)
        AND(parent.m_11 = child.m_12)
    WHERE parent.Id = @parentId
    ORDER By siblingId DESC
GO

group-by 之前没有工作，因为我无法从子查询中检索 m_12 和 m_22，因为我没有 group by 两个值.切换到 left-join 会导致报告空值，这正是我所需要的！

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