We all know about the foreign key reference. But what happens when we want foreign key references across the Database? This article shows you how. Hope it will be informative.

To understand it properly, we are trying to make a pictorial diagram.

Here we have a Microsoft SQL Server named DB Server that has two Database named DB1 and DB2. The DB1 database has a table named Table1 and DB2 database has a table named Table2. We are trying to make the Foreign Key relation between the Table1 and Table2.

Let’s Take an Example to Understand It

Step-1 [ Creating the base table ]

 USE [DB1];
GO

IF OBJECT_ID(N'[dbo].[tbl_ITEM]', N'U')IS NOT NULL
   BEGIN
       DROP TABLE [dbo].[tbl_ITEM];
   END
GO

CREATE TABLE [dbo].[tbl_ITEM]
       (
          ITEMCODE   INT             NOT NULL PRIMARY KEY,
          ITEMMNAME  VARCHAR(50)     NOT NULL
       );
GO

INSERT INTO [dbo].[tbl_ITEM]
       (ITEMCODE, ITEMMNAME)
VALUES (101, 'Tooth Paste'),
       (102, 'Tooth Brush'),
       (103, 'Saving Lootion');
GO

ITEMCODE    ITEMMNAME
----------- --------------------------------------------------
101         Tooth Paste
102         Tooth Brush
103         Saving Lootion

(3 row(s) affected)

USE [DB2];
GO

IF OBJECT_ID(N'[dbo].[tbl_ITEMORDER]', N'U')IS NOT NULL
   BEGIN
       DROP TABLE [dbo].[tbl_ITEMORDER];
   END
GO

CREATE TABLE [dbo].[tbl_ITEMORDER]
  (
    ORDERNO    INT            NOT NULL IDENTITY PRIMARY KEY,   
    ITEMCODE   INT            NOT NULL,
    QTY        DECIMAL(18,2)  NOT NULL
  );
GO

Step-2 [ Try to Creating the Foreign Key Relation ]

 ALTER TABLE [dbo].[tbl_ITEMORDER]
ADD CONSTRAINT FK_ITEMCODE_tbl_ITEMORDER FOREIGN KEY(ITEMCODE)
REFERENCES [DB1].[dbo].[tbl_ITEM](ITEMCODE);

It gives the error:

 Msg 1763, Level 16, State 0, Line 1
Cross-database foreign key references are not supported. Foreign key 'DB1.dbo.tbl_ITEM'.
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.

So the Cross Database Foreign key Relation is not possible Directly.

Step-3 [ What's the Solution ]

We can do the something by CHECK Constraint. Here the example is.

First we create a Function:

 IF OBJECT_ID(N'[dbo].[func_CHECK_ITEMCODEREFERENCE]', N'FN')IS NOT NULL
   BEGIN
       DROP FUNCTION [dbo].[func_CHECK_ITEMCODEREFERENCE];
   END
GO

CREATE FUNCTION [dbo].[func_CHECK_ITEMCODEREFERENCE]
       (
          @p_ITEMCODE   INT = 0
       )
RETURNS INT
AS
BEGIN
    DECLARE @v_RetVal   INT;
     SET @v_RetVal = 0;

     IF EXISTS(SELECT *
                FROM [DB1].[dbo].[tbl_ITEM]
                WHERE ITEMCODE = @p_ITEMCODE)
        BEGIN
           SET @v_RetVal = 1;
        END

    RETURN @v_RetVal;
END

Use the function in Alter statement CHECK Constraint.

 ALTER TABLE [dbo].[tbl_ITEMORDER]
ADD CONSTRAINT CHECK_ITEMCODE_tbl_ITEMORDER
CHECK([dbo].[func_CHECK_ITEMCODEREFERENCE](ITEMCODE)=1);

Step-4 [ Now Check it ]

 INSERT INTO [dbo].[tbl_ITEMORDER]
       (ITEMCODE, QTY)
VALUES (101, 200);

It inserted perfectly as the ITEMCODE 101 is present in the Table TBL_ITEM in the Database DB1.

 INSERT INTO [dbo].[tbl_ITEMORDER]
       (ITEMCODE, QTY)
VALUES (110, 200);

Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint "CHECK_ITEMCODE_tbl_ITEMORDER". The conflict occurred in database "DB2", table "dbo.tbl_ITEMORDER", column 'ITEMCODE'.
The statement has been terminated.

So it is working like Foreign Key Constraint and hence the Solution is.

Hope you like it.