当表引用自身时，如何编写查找所有循环引用的查询？

我有以下架构（名称已更改），无法更改：

CREATE TABLE MyTable (
    Id INT NOT NULL PRIMARY KEY,
    ParentId INT NOT NULL
);

ALTER TABLE MyTable ADD FOREIGN KEY (ParentId) REFERENCES MyTable(Id);

也就是说，每个记录都是另一个记录的子级。如果一条记录ParentId等于它的Id，则该记录被视为根节点。

我想运行查询，将找到所有循环引用。例如，与数据

INSERT INTO MyTable (Id, ParentId) VALUES
    (0, 0),
    (1, 0),
    (2, 4),
    (3, 2),
    (4, 3);

查询应返回

Id | Cycle
2  | 2 < 4 < 3 < 2
3  | 3 < 2 < 4 < 3
4  | 4 < 3 < 2 < 4

我为SQL Server 2008 R2编写了以下查询，我想知道是否可以改进此查询：

IF OBJECT_ID(N'tempdb..#Results') IS NOT NULL DROP TABLE #Results;
CREATE TABLE #Results (Id INT, HasParentalCycle BIT, Cycle VARCHAR(MAX));

DECLARE @i INT,
    @j INT,
    @flag BIT,
    @isRoot BIT,
    @ids VARCHAR(MAX);

DECLARE MyCursor CURSOR FAST_FORWARD FOR
    SELECT Id
    FROM MyTable;

OPEN MyCursor;
FETCH NEXT FROM MyCursor INTO @i;
WHILE @@FETCH_STATUS = 0
BEGIN
    IF OBJECT_ID(N'tempdb..#Parents') IS NOT NULL DROP TABLE #Parents;
    CREATE TABLE #Parents (Id INT);

    SET @ids = NULL;
    SET @isRoot = 0;
    SET @flag = 0;
    SET @j = @i;
    INSERT INTO #Parents (Id) VALUES (@j);

    WHILE (1=1)
    BEGIN
        SELECT
            @j = ParentId,
            @isRoot = CASE WHEN ParentId = Id THEN 1 ELSE 0 END
        FROM MyTable
        WHERE Id = @j;

        IF (@isRoot = 1)
        BEGIN
            SET @flag = 0;
            BREAK;
        END        

        IF EXISTS (SELECT 1 FROM #Parents WHERE Id = @j)
        BEGIN
            INSERT INTO #Parents (Id) VALUES (@j);
            SET @flag = 1;
            SELECT @ids = COALESCE(@ids + ' < ', '') + CAST(Id AS VARCHAR) FROM #Parents;
            BREAK;
        END
        ELSE
        BEGIN
            INSERT INTO #Parents (Id) VALUES (@j);
        END        
    END

    INSERT INTO #Results (Id, HasParentalCycle, Cycle) VALUES (@i, @flag, @ids);

    FETCH NEXT FROM MyCursor INTO @i;
END
CLOSE MyCursor;
DEALLOCATE MyCursor;

SELECT Id, Cycle
FROM #Results
WHERE HasParentalCycle = 1;

这需要递归CTE：

WITH FindRoot AS
(
    SELECT Id,ParentId, CAST(Id AS NVARCHAR(MAX)) Path
    FROM dbo.MyTable

    UNION ALL

    SELECT C.Id, P.ParentId, C.Path + N' > ' + CAST(P.Id AS NVARCHAR(MAX))
    FROM dbo.MyTable P
    JOIN FindRoot C
    ON C.ParentId = P.Id AND P.ParentId <> P.Id AND C.ParentId <> C.Id
 )
SELECT *
FROM FindRoot R
WHERE R.Id = R.ParentId 
  AND R.ParentId <> 0;

在此处查看其运行情况：SQL Fiddle

更新：

增加了距离以能够排除所有自循环（请参见ypercube的注释）：

WITH FindRoot AS
(
    SELECT Id,ParentId, CAST(Id AS NVARCHAR(MAX)) Path, 0 Distance
    FROM dbo.MyTable

    UNION ALL

    SELECT C.Id, P.ParentId, C.Path + N' > ' + CAST(P.Id AS NVARCHAR(MAX)), C.Distance + 1
    FROM dbo.MyTable P
    JOIN FindRoot C
    ON C.ParentId = P.Id AND P.ParentId <> P.Id AND C.ParentId <> C.Id
 )
SELECT *
FROM FindRoot R
WHERE R.Id = R.ParentId 
  AND R.ParentId <> 0
  AND R.Distance > 0;

SQL小提琴

您应该使用哪一个取决于您的要求。

SELECT RC.CONSTRAINT_NAME FK_Name
, KF.TABLE_SCHEMA FK_Schema
, KF.TABLE_NAME FK_Table
, KF.COLUMN_NAME FK_Column
, RC.UNIQUE_CONSTRAINT_NAME PK_Name
, KP.TABLE_SCHEMA PK_Schema
, KP.TABLE_NAME PK_Table
, KP.COLUMN_NAME PK_Column
, RC.MATCH_OPTION MatchOption
, RC.UPDATE_RULE UpdateRule
, RC.DELETE_RULE DeleteRule
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS RC
JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE KF ON RC.CONSTRAINT_NAME = KF.CONSTRAINT_NAME
JOIN INFORMATION_SCHEMA.KEY_COLUMN_USAGE KP ON RC.UNIQUE_CONSTRAINT_NAME = KP.CONSTRAINT_NAME
WHERE KF.TABLE_NAME = KP.TABLE_NAME