Unable to create a FK constraint

Question

I have 2 tables:

TableA
------
TableAID INT IDENTITY Primary Key
TableBID INT NULL

I add a unique constraint on TableBID
ALTER TABLE TableAID ADD CONSTRAINT xx UNIQUE ([TableBID])

TableB
------
TableBID INT IDENTITY Primary Key

Now when I delete TableA row, I need TableB's to be deleted as well, so i am trying to add a FK with cascade delete

ALTER TABLE TableB ADD CONSTRAINT yy FOREIGN KEY (TableBID) 
REFERENCES TableA (TABLEBID) ON DELETE CASCADE

But I get this error:

Msg 1788, Level 16, State 0, Line 1
Cascading foreign key 'yy' cannot be created where the referencing column 'TableB.TableBID' is an identity column.

Answer 1

TableB.TableBID cannot simultaneously be an IDENTITY column (which is calcluated independently) and at the same time be a foreign key from a column in another table.

I think you've got your relationship reversed and actually want:

ALTER TABLE TableA ADD CONSTRAINT yy FOREIGN KEY (TableBID) 
REFERENCES TableB (TABLEBID) ON DELETE CASCADE