This article was previously published under Q321843
You may receive the following error message when you create a FOREIGN KEY constraint:
Server: Msg 1785, Level 16, State 1, Line 1Introducing FOREIGN KEY constraint 'fk_two' on table 'table2' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.Server: Msg 1750, Level 16, State 1, Line 1Could not create constraint. See previous errors.
You receive this error message because in SQL Server, a table cannot appear more than one time in a list of all the cascading referential actions that are started by either a DELETE or an UPDATE statement. For example, the tree of cascading referential actions must only have one path to a particular table on the cascading referential actions tree.
To work around this problem, do not create a foreign key that will create more than one path to a table in a list of cascading referential actions.
You can enforce referential integrity in several ways. Declarative Referential Integrity (DRI) is the most basic way, but it is also the least flexible way. If you need more flexibility, but you still want a high degree of integrity, you can use triggers instead.
The following sample code is an example of a FOREIGN KEY creation attempt that generates the error message:
Use tempdbgocreate table table1 (user_ID integer not null primary key, user_namechar(50) not null)gocreate table table2 (author_ID integer not null primary key, author_namechar(50) not null, lastModifiedBy integer not null, addedby integer notnull)goalter table table2 add constraint fk_one foreign key (lastModifiedby)references table1 (user_ID) on delete cascade on update cascadegoalter table table2 add constraint fk_two foreign key (addedby)references table1(user_ID) on delete no action on update cascadego--this fails with the error because it provides a second cascading path to table2.alter table table2 add constraint fk_two foreign key (addedby)references table1 (user_ID) on delete no action on update no actiongo-- this works.
For more information, please see the "Cascading Referential Integrity Constraints" topic in SQL Server 2000 Books Online.