是否可以将非主键设置为另一个表中的外键？

Question

More specifically, I would like to set the Email column from AspNetUsers table as foreign key in another table.

I hope to have explained my problem well.

Answer 1

Yes and No

SQL Server, specifically:

If you're just asking about SQL Server (and not Entity Framework), then I quoteth Microsoft's documentation :

A foreign key constraint doesn't have to be linked only to a primary key constraint in another table. Foreign keys can also be defined to reference the columns of a UNIQUE constraint in another table.

Therefore to answer your stated question directly:

• Is it possible to set a non primary key as foreign key in another table?
  • If they're "normal" columns, then no, because otherwise SQL Server has no way of knowing that those values will uniquely identify each row.
  • But if the columns are in either in a UNIQUE CONSTRAINT or have a non-filtered UNIQUE INDEX applied to them, then yes , you can.
  • This requires SQL Server 2005 or later. (I might be wrong, but I don't believe SQL Server 2000 supported FOREIGN KEY constraint columns referencing UNIQUE CONSTRAINT columns.

There are subtle differences between UNIQUE CONSTRAINT and UNIQUE INDEX , though the main difference is indicating intent - so you should prefer UNIQUE CONSTRAINT over UNIQUE INDEX when you're using a column - or columns - as a secondary key .

Entity Framework

• Entity Framework 6 :

  • I'll admit I'm unsure. I remember having to really fight EF6 and use workarounds like VIEW s to make EF6 play-nice with FK references to UNIQUE KEY constraints, but some reports suggest that modern versions of EF6 do support this natively.

• Entity Framework Core :

  • Yes. Use HasPrincipal() with HasForeignKey() to define both a secondary-key on TPrincipal and the foreign-key on TDependant that points to that secondary-key.

---

As for your scenario, specifically: dbo.AspNetUsers :

Assuming that you want to use the Email column to uniquely identify users, then I recommend that you don't do that : Generally speaking, it's a bad idea to use mutable data (ie data that can change) as a key. Keys must be unique and ideally should be immutable - and while an e-mail address can be unique in some contexts (eg your system won't allow multiple users to share the same e-mail address) they aren't immutable (users can and do change e-mail addressees).

Another caveat: in order to use a UNIQUE CONSTRAINT and UNIQUE INDEX as a foreign-key target then those column(s) cannot be filtered or have NULL s - but dbo.AspNetUser.Email is nvarchar(256) NULL , so you'll need to ensure all users have distinct email addresses set first .

Having said that, this will work:

-- 1. Change `Email` from `NULL` to a `NOT NULL` column, as `UNIQUE CONSTRAINT` columns cannot contain duplicates or nulls:
ALTER TABLE dbo.AspNetUsers
    ALTER COLUMN [Email] nvarchar(256) NOT NULL;

-- 2. Create a UNIQUE CONSTRAINT over the `Email` column, so SQL Server knows every row has a distinct value in that column:
ALTER TABLE dbo.AspNetUsers
    ADD CONSTRAINT UQ_Email UNIQUE ( Email );

-- 3. Create a new table with a FOREIGN KEY in the other table:
CREATE TABLE dbo.OtherTable (
    Etc       nvarchar(50) NOT NULL,
    UserEmail nvarchar(256) NULL,
    
    CONSTRAINT FK_Users_by_email FOREIGN KEY ( UserEmail ) REFERENCES dbo.AspNetUsers ( [Email] )
);

Answer 2

Yes you can make foreign key by principal key in asp.net core, by using fluent api. You can see an example from microsoft doc from here

Answer 3

i was able to create a foreign key to a non-pk column. no issues.

CREATE TABLE [dbo].[t1](
[id] [int] NOT NULL,
[fldKey] [int] NULL,
CONSTRAINT [PK_t1] PRIMARY KEY CLUSTERED 
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

CREATE TABLE [dbo].[t2](
[id] [int] NOT NULL,
[t1] [int] NULL,
CONSTRAINT [PK_t2] PRIMARY KEY CLUSTERED 
(
[id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[t2]  WITH CHECK ADD  CONSTRAINT [FK_t2_t1] FOREIGN 
KEY([t1])
REFERENCES [dbo].[t1] ([fldKey])
GO
ALTER TABLE [dbo].[t2] CHECK CONSTRAINT [FK_t2_t1]
GO