可空外键约束

Question

We are trying increase the scope of database compatibility for our web application. Our application is Java EE with JSPs, Servlets, and EJBs. The database we are trying to make our application compatible with is SQL Server 2008.

The problem we are running into is that our application uses nullable foreign keys in many cases in many files. These nullable foreign keys work in other databases but we have not found a way to get them to work in SQL Server 2008 because it will only allow a single foreign key to be 'null' at a given time. We understand that, in general, it is best to avoid such nullable foreign keys. However, this web application is large and it would be quite difficult to change files one-by-one.

Thus far, we have tried the following:
[1] initializing dummy elements in referenced tables so that the foreign keys will point to something.
[2] using 'EXEC sp_msforeachtable "ALTER TABLE ? NOCHECK CONSTRAINT all"' in order to remove the foreign key constraint

Unfortunately, initializing dummy elements in [1] above broke many components in the web application.

Unfortunately, trying to remove the constraint according to the statement in [2] above did not work. We suspect this because subsequent attempts to drop each table result in Foreign Key constraint errors.

Currently, clear answers to the following questions would help us make some progress:
[1] Is there a quick fix to allow SQL Server 2008 to allow for multiple 'null'-valued foreign keys?
[2] Is there another workaround we could try that would not involve extensive changes to our web application?

Answer 1

I'm not sure what you are talking about! Possibly if you posted a sample schema, I could understand what you mean.

You can have multiple null FK columns in a table :

build the tables and FKs:

CREATE TABLE dbo.AAAA
    (
    A_ID int NOT NULL identity(1,1) primary key,
    B_ID int NULL,
    C_ID int NULL
    )  ON [PRIMARY]

CREATE TABLE dbo.BBBB
    (
    B_ID int NOT NULL identity(1,1) primary key,
    A_ID int NULL,
    C_ID int NULL
    )  ON [PRIMARY]

CREATE TABLE dbo.CCCC
    (
    C_ID int NOT NULL identity(1,1) primary key,
    A_ID int NULL,
    B_ID int NULL
    )  ON [PRIMARY]


ALTER TABLE dbo.CCCC ADD CONSTRAINT FK_CCCC_AAAA FOREIGN KEY ( A_ID ) REFERENCES dbo.AAAA ( A_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 
ALTER TABLE dbo.BBBB ADD CONSTRAINT FK_BBBB_AAAA FOREIGN KEY ( A_ID ) REFERENCES dbo.AAAA ( A_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 
ALTER TABLE dbo.CCCC ADD CONSTRAINT FK_CCCC_BBBB FOREIGN KEY ( B_ID ) REFERENCES dbo.BBBB ( B_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 
ALTER TABLE dbo.AAAA ADD CONSTRAINT FK_AAAA_BBBB FOREIGN KEY ( B_ID ) REFERENCES dbo.BBBB ( B_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 
ALTER TABLE dbo.AAAA ADD CONSTRAINT FK_AAAA_CCCC FOREIGN KEY ( C_ID ) REFERENCES dbo.CCCC ( C_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 
ALTER TABLE dbo.BBBB ADD CONSTRAINT FK_BBBB_CCCC FOREIGN KEY ( C_ID ) REFERENCES dbo.CCCC ( C_ID ) ON UPDATE  NO ACTION  ON DELETE  NO ACTION 

insert sample data:

INSERT INTO AAAA VALUES (NULL,NULL)
INSERT INTO AAAA VALUES (NULL,NULL)
INSERT INTO AAAA VALUES (NULL,NULL)

INSERT INTO BBBB VALUES (1,NULL)
INSERT INTO BBBB VALUES (2,NULL)
INSERT INTO BBBB VALUES (NULL,NULL)
INSERT INTO BBBB VALUES (NULL,NULL)
INSERT INTO BBBB VALUES (1,NULL)

Show the data (see how many FK columns are null):

select * from AAAA
select * from BBBB
select * from CCCC

OUTPUT:

A_ID        B_ID        C_ID
----------- ----------- -----------
1           NULL        NULL
2           NULL        NULL
3           NULL        NULL

(3 row(s) affected)

B_ID        A_ID        C_ID
----------- ----------- -----------
1           1           NULL
2           2           NULL
3           NULL        NULL
4           NULL        NULL
5           1           NULL

(5 row(s) affected)

C_ID        A_ID        B_ID
----------- ----------- -----------

(0 row(s) affected)

If this isn't what you are talking about, you need to provide some sample tables and data.

remove these test tables:

ALTER TABLE dbo.CCCC drop CONSTRAINT FK_CCCC_AAAA
ALTER TABLE dbo.BBBB drop CONSTRAINT FK_BBBB_AAAA
ALTER TABLE dbo.CCCC drop CONSTRAINT FK_CCCC_BBBB
ALTER TABLE dbo.AAAA drop CONSTRAINT FK_AAAA_BBBB
ALTER TABLE dbo.AAAA drop CONSTRAINT FK_AAAA_CCCC
ALTER TABLE dbo.BBBB drop CONSTRAINT FK_BBBB_CCCC
drop table AAAA
drop table BBBB
drop table CCCC