Foreign key Relation Across Database
Introduction
As we all know about the foreign key reference. But what happens when we want foreign key references across the Database. This article is related to it. Hope, it will be informative.
Understand the Case
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 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 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')ISNOT 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.
Foreign Key Refers the Columns of Same Table
Introduction
We all know about the Foreign Key constraint. Here in this article, we are not going to discuss about the definition or implementation concept of Foreign Key. But here we are trying to discuss about the Foreign key that refers the another a column of same table.
What is That?
EMPID
|
EMPNAME
|
DESIGNATION
|
MANAGERID
|
101
|
Sudip Das
|
Manager
|
101
|
102
|
Joydeep Das
|
Group Lead
|
101
|
103
|
Sukamal Jana
|
Group Lead
|
105
|
Here EPID is the Primary Key and the MANAHERID is the foreign key which refers the EMPID of the same table.
In the above situation Employee ID 101 is in manager position, so its Manager ID is same 101. But for Employee ID 102 the Manager ID is 101 and we can insert data without any error.
But in case of Employee ID 103 if we provide the Manager ID 105 it gives us an error as no such employee id (105) is not present and Foreign key gives us an Error over there.
How we implement That
Step-1 [ The Base Table with Foreign Key References ]
IF OBJECT_ID(N'dbo.tbl_EMPLOYEEMASTER', N'U')IS NOT NULL
BEGIN
DROP TABLE [dbo].[tbl_EMPLOYEEMASTER];
END
GO
CREATE TABLE [dbo].[tbl_EMPLOYEEMASTER]
(
EMPID INT NOT NULL PRIMARY KEY,
EMPNAME VARCHAR(50) NOT NULL,
DESIGNATION VARCHAR(50) NOT NULL,
MANAGERID INT NOT NULL,
CONSTRAINT FK_MANAGERID_tbl_EMPLOYEEMASTER FOREIGN KEY(MANAGERID)
REFERENCES [dbo].[tbl_EMPLOYEEMASTER](EMPID)
);
Here just look at the definition of foreign key specially the REFERENCES section.
Step-2 [ Insert Firs Record ]
INSERT INTO [dbo].[tbl_EMPLOYEEMASTER]
(EMPID, EMPNAME, DESIGNATION, MANAGERID)
VALUES (101, 'Sudip Das', 'Manager', 101);
GO
Here we not find any error as Manager ID is the Same as the Employee ID and Foreign key Satisfied.
Step-3 [ Insert Second Record ]
INSERT INTO [dbo].[tbl_EMPLOYEEMASTER]
(EMPID, EMPNAME, DESIGNATION, MANAGERID)
VALUES (102, 'Joydeep Das', 'Group Lead', 101);
GO
Here we do not get any error as Manager ID 101 is present in the table and Foreign key satisfied.
Step-4 [ Insert Third Record ]
INSERT INTO [dbo].[tbl_EMPLOYEEMASTER]
(EMPID, EMPNAME, DESIGNATION, MANAGERID)
VALUES (103, 'Sukamal Jana', 'Group Lead', 104);
GO
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the FOREIGN KEY SAME TABLE constraint "FK_MANAGERID_tbl_EMPLOYEEMASTER". The conflict occurred in database "PRACTICE_DB", table "dbo.tbl_EMPLOYEEMASTER", column 'EMPID'.
The statement has been terminated.
Here it gives error as Manager ID 104 is not present in the Table.
Hope you like it.
Is Conditional Foreign Key is Possible
Introduction
One of my friends wants a conditional foreign key. I suppressed that is possible? Before proceed let’s see the case study.
Case Study
We have an Employee Master Table calledtbl_EMPMASTER which contains designation for both team lead and programmer.
Now we are going to create two children Table and use foreign key with master table. We want the Team Lead (TL) designation holder stores in table called tbl_EMP_TLand the Programmer (PRG) designation holder stores on tbl_EMP_PRG.
We must use foreign key for referential integrity.
Here is the Pictorial Diagram of Case study
Solutions
/*
Logical Conditional Foreign Key
*/
IF OBJECT_ID(N'dbo.tbl_EMPMASTER', N'U') IS NOT NULL
BEGIN
DROP TABLE dbo.tbl_EMPMASTER;
END
GO
CREATE TABLE dbo.tbl_EMPMASTER
(
EMPID INT NOT NULL PRIMARY KEY,
EMPNAME VARCHAR(50) NOT NULL,
EMPDESIG VARCHAR(10) NOT NULL,
EMPDSGSTAT AS CASE WHEN EMPDESIG='TL' THEN 1
WHEN EMPDESIG='PRG' THEN 2
END PERSISTED,
CONSTRAINT UK_EMPID_EMPDSGSTAT UNIQUE(EMPID, EMPDSGSTAT)
);
-- Inserting Master Data
INSERT INTO dbo.tbl_EMPMASTER
(EMPID, EMPNAME, EMPDESIG)
VALUES (1, 'Joydeep Das', 'TL'),
(2, 'Sangram Jit Bhattacharya', 'PRG');
GO
IF OBJECT_ID(N'dbo.tbl_EMP_TL', N'U') IS NOT NULL
BEGIN
DROP TABLE dbo.tbl_EMP_TL;
END
GO
CREATE TABLE dbo.tbl_EMP_TL
(EMPID INT NOT NULL PRIMARY KEY,
EMPSAL DECIMAL(19,2) NOT NULL,
EMPTLSTAT INT CHECK(EMPTLSTAT = 1),
FOREIGN KEY(EMPID,EMPTLSTAT) REFERENCESdbo.tbl_EMPMASTER(EMPID,EMPDSGSTAT));
-- Insering Data For TL Only
INSERT INTO dbo.tbl_EMP_TL
(EMPID, EMPSAL, EMPTLSTAT)
VALUES (1, 2000, 1);
(1 row(s) affected)
INSERT INTO dbo.tbl_EMP_TL
(EMPID, EMPSAL, EMPTLSTAT)
VALUES (2, 1000, 1);
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the FOREIGN KEY constraint
"FK__tbl_EMP_TL__1A9D589D". The conflict occurred
in database "TEST", table "dbo.tbl_EMPMASTER".
The statement has been terminated.
GO
IF OBJECT_ID(N'dbo.tbl_EMP_PRG', N'U') IS NOT NULL
BEGIN
DROP TABLE dbo.tbl_EMP_PRG;
END
GO
CREATE TABLE dbo.tbl_EMP_PRG
(
EMPID INT NOT NULL PRIMARY KEY,
EMPSAL DECIMAL(19,2) NOT NULL,
EMPTLSTAT INT CHECK(EMPTLSTAT = 2),
FOREIGN KEY(EMPID,EMPTLSTAT) REFERENCESdbo.tbl_EMPMASTER(EMPID,EMPDSGSTAT)
);
GO
INSERT INTO dbo.tbl_EMP_PRG
(EMPID, EMPSAL, EMPTLSTAT)
VALUES (2, 1000, 2);
(1 row(s) affected)
INSERT INTO dbo.tbl_EMP_PRG
(EMPID, EMPSAL, EMPTLSTAT)
VALUES (1, 2000, 2);
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the FOREIGN KEY constraint
"FK__tbl_EMP_PRG__260F0B49". The conflict occurred
in database "TEST", table "dbo.tbl_EMPMASTER".
The statement has been terminated.
