--SQLServer 递归查询，主要用于建立树形结构

IF OBJECT_ID('Categories') IS NOT NULL
    DROP TABLE Categories
GO

CREATE TABLE Categories (
    CategoryID INT, 
    CategoryName VARCHAR(20),
    ParentID INT )
GO

INSERT INTO Categories(CategoryID, CategoryName, ParentID)
    SELECT 1, 'Books', NULL UNION ALL
    SELECT 2, 'SQL Server', 1 UNION ALL
    SELECT 3, 'ASP.NET', 1 UNION ALL
    SELECT 4, 'General', 2 UNION ALL
    SELECT 5, 'SSIS', 2 UNION ALL
    SELECT 6, 'TSQL', 2 UNION ALL
    SELECT 7, 'SSRS', 2 UNION ALL
    SELECT 8, 'Beginners', 4 UNION ALL
    SELECT 9, 'Certification Guide', 4
go

WITH cte AS (
    SELECT 0 AS lvl, CategoryID, CategoryName, ParentID,
        CAST(CategoryID AS VARCHAR(128)) AS Sort
    FROM Categories WHERE ParentID IS NULL
    UNION ALL
    SELECT p.lvl + 1, c.CategoryID, c.CategoryName, c.ParentID,
        CAST(p.Sort + '/' + CAST(c.CategoryID AS VARCHAR) AS VARCHAR

(128))
    FROM Categories c
    INNER JOIN cte p ON p.CategoryID = c.ParentID
)
SELECT 
    CategoryID, 
    SPACE(lvl * 4) + CategoryName AS CategoryName, 
    Sort,
    ParentID 
FROM cte
ORDER BY Sort