Select 不使用 CTE 的分层数据

Question

我有以下 table:

CREATE TABLE [dbo].[ProductHierarchy]
(
    [ProductHierarchyID] INT NOT NULL
          PRIMARY KEY CLUSTERED IDENTITY(1, 1),
    [ProductID] INT NOT NULL ,
    [ParentProductID] INT NULL
);

使用此数据：

INSERT INTO [dbo].[ProductHierarchy] ([ProductID], [ParentProductID])
VALUES (1, NULL), (2, 1), (3, 1), (4, 2), (5, 4), (6, 4), (7, 4);

现在，我可以像这样使用 CTE 进行分层查询：

WITH [CTE_Products] ([ProductID], [ParentProductID], [ProductLevel]) AS 
(
    SELECT
        [ProductID],
        [ParentProductID],
        0 AS [ProductLevel]
    FROM 
        [dbo].[ProductHierarchy]
    WHERE 
        [ParentProductID] IS NULL

    UNION ALL

    SELECT  
        [pn].[ProductID],
        [pn].[ParentProductID],
        [p1].[ProductLevel] + 1
    FROM 
        [dbo].[ProductHierarchy] AS [pn]
    INNER JOIN 
        [CTE_Products] AS [p1] ON [p1].[ProductID] = [pn].[ParentProductID]
)
SELECT  
    [ProductID],
    [ParentProductID],
    [ProductLevel]
FROM 
    [CTE_Products]
ORDER BY 
    [ParentProductID];

但是，如何在不使用 CTE 的情况下通过一个 SQL 语句达到相同的目标？这可能吗？

Answer 1

CTE 允许解决这种递归查询。如果你提前知道深度级别，你可以 "unwind" 递归部分：

SELECT p0.ProductId, p0.ParentProductID, 0 AS ProductLevel
FROM [dbo].[ProductHierarchy] p0
WHERE p0.ParentProductId IS NULL
UNION ALL
SELECT p1.ProductId, p1.ParentProductID, 1 AS ProductLevel
FROM [dbo].[ProductHierarchy] p1
JOIN (SELECT p0.ProductId, p0.ParentProductID, 0 AS ProductLevel
      FROM [dbo].[ProductHierarchy] p0
      WHERE p0.ParentProductId IS NULL) p0
  ON p1.ParentProductId = p0.ProductId
UNION ALL
SELECT p2.ProductId, p2.ParentProductID, 2 AS ProductLevel
FROM [dbo].[ProductHierarchy] p2
JOIN (SELECT p1.ProductId, p1.ParentProductID, 1 AS ProductLevel
      FROM [dbo].[ProductHierarchy] p1
      JOIN (SELECT p0.ProductId, p0.ParentProductID, 0 AS ProductLevel
      FROM [dbo].[ProductHierarchy] p0
      WHERE p0.ParentProductId IS NULL) p0
        ON p1.ParentProductId = p0.ProductId) p1
  ON p2.ParentProductId = p1.ProductId
UNION ALL
SELECT p3.ProductId, p3.ParentProductID, 3 AS ProductLevel
FROM [dbo].[ProductHierarchy] p3
JOIN  (SELECT p2.ProductId, p2.ParentProductID, 2 AS ProductLevel
       FROM [dbo].[ProductHierarchy] p2
       JOIN (SELECT p1.ProductId, p1.ParentProductID, 1 AS ProductLevel
             FROM [dbo].[ProductHierarchy] p1
             JOIN (SELECT p0.ProductId, p0.ParentProductID, 0 AS ProductLevel
                    FROM [dbo].[ProductHierarchy] p0
                    WHERE p0.ParentProductId IS NULL) p0
              ON p1.ParentProductId = p0.ProductId) p1
        ON p2.ParentProductId = p1.ProductId) p2
  ON p3.ParentProductId = p2.ProductId;

DBFiddle Demo

Select 不使用 CTE 的分层数据

Select hierarchical data without using CTE

sql

tsql

sql-server

hierarchical-data