sql - 在不使用 CTE 的情况下选择分层数据
问题描述
我有下表:
CREATE TABLE [dbo].[ProductHierarchy]
(
[ProductHierarchyID] INT NOT NULL
PRIMARY KEY CLUSTERED IDENTITY(1, 1),
[ProductID] INT NOT NULL ,
[ParentProductID] INT NULL
);
有了这些数据:
INSERT INTO [dbo].[ProductHierarchy] ([ProductID], [ParentProductID])
VALUES (1, NULL), (2, 1), (3, 1), (4, 2), (5, 4), (6, 4), (7, 4);
现在,我可以使用这样的 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 的情况下使用一条 SQL 语句达到相同的目标呢?这可能吗?
解决方案
CTE 允许解决这种递归查询。如果您提前知道深度级别,则可以“展开”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;
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