SQL Server 2008-添加具有约束的自引用外键并获得“找不到数据类型”错误

Question

I am working in SQL Server 2008 and am trying to add a self referencing foreign key with an ON DELETE CASCADE constraint but am running into some issues. Basically I want the system to automatically delete all children of a node all the way down the hierarchy when a node is deleted.

I have a table, Node with the following basic structure:

• PK_Node_Id (primary key)
• ParentNodeId (want to make this the self-referencing foreign key with ON DELETE CASCADE constraint.

The problem lies when I try to add the constraint. If I use the GUI Designer, the INSERT and UPDATE Specification selections are grayed out and I can't do anything.

If I try to run an SQL statement to add the constraint:

ALTER TABLE Node
ADD CONSTRAINT
FK_Node_Node
FOREIGN KEY (ParentNodeId)
REFERENCES Node (PK_Node_Id)
ON DELETE CASCADE;

I get the following error:

Cannot find data type FK_Node_Node.

FK_Node_Node is the name of my foreign key. Not sure why it is asking for or looking for a datatype...

Thanks for your help!

Answer 1

I get the following error: Cannot find data type FK_Node_Node.

I suspect you misspelled the word "CONSTRAINT" when you ran the ALTER TABLE statement. The resulting statement is still syntactically valid, but instead of adding a foreign key constraint, it adds a column named (for example) "CONSTAINT" of type FK_Node_Node, plus an unnamed foreign key constraint.

I want the system to automatically delete all children of a node all the way down the hierarchy when a node is deleted.

Unfortunately, this isn't possible: SQL Server doesn't allow cascading referential actions to form cycles. The error the ALTER TABLE statement should have given you is:

Introducing FOREIGN KEY constraint 'FK_Node_Node' on table 'Node' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

Instead of using ON DELETE CASCADE, you need to perform the recursive deletion yourself:

;WITH Nodes (PK_Node_Id) AS
(
    SELECT PK_Node_Id
    FROM Node
    WHERE PK_Node_Id = @NodeId  -- @NodeId is the root node you want to delete

    UNION ALL

    SELECT Child.PK_Node_Id
    FROM Node Child JOIN Nodes Parent ON Parent.PK_Node_Id = Child.ParentNodeId
)
DELETE FROM Node
WHERE PK_Node_Id IN (SELECT PK_Node_Id FROM Nodes)