SQL with Manoj

Just replied on MSDN forum’s following post: http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/289c4ca5-6ab2-4ed6-b446-0300f5c7dc88

When tables are referenced by Foreign Key constraints it gets difficult to UPDATE the key columns.
An example shows this issue and how one can perform UPDATE on FK columns.

USE [tempdb]
GO

-- CREATE test tables
CREATE TABLE dept (d_id INT PRIMARY KEY, dept VARCHAR(50))

CREATE TABLE emp (sn INT PRIMARY KEY, ename VARCHAR(50),
	d_fk INT CONSTRAINT fk_dept_d_fk FOREIGN KEY (d_fk) REFERENCES dept(d_id))

-- INSERT test data
INSERT INTO dept
SELECT 1, 'finance' UNION
SELECT 2, 'hr'

INSERT INTO emp
SELECT 1, 'Manoj', 1 UNION
SELECT 2, 'Saurabh', 2 UNION
SELECT 3, 'Kanchan', 1 UNION
SELECT 4, 'Pooja', 2

-- SELECT the INSERTed data
SELECT * from dept

d_id	dept
1	finance
2	hr	-- DeptID needs to be updated to 3

SELECT * from emp

sn	ename	d_fk
1	Manoj	1
2	Saurabh	2	-- DeptID should also be updated here to 3
3	Kanchan	1
4	Pooja	2	-- DeptID should also be updated here to 3

-- Now lets UPDATE the FK column values
UPDATE dept SET d_id=3 WHERE d_id=2

Error Message:
Msg 547, Level 16, State 0, Line 1
The UPDATE statement conflicted with the REFERENCE constraint "fk_dept_d_fk".
The conflict occurred in database "tempdb", table "dbo.emp", column 'd_fk'.
The statement has been terminated.

UPDATE emp SET d_fk=3 WHERE d_fk=2

Error Message:
Msg 547, Level 16, State 0, Line 1
The UPDATE statement conflicted with the FOREIGN KEY constraint "fk_dept_d_fk".
The conflict occurred in database "tempdb", table "dbo.dept", column 'd_id'.
The statement has been terminated.

We can disable the FK constraints by ALTER statement before the UPDATE and enable them after the UPDATE, like:

-- Disable FK Constraint
ALTER TABLE emp NOCHECK CONSTRAINT fk_dept_d_fk

-- Perform UPDATE
UPDATE dept SET d_id=3 WHERE d_id=2
UPDATE emp SET d_fk=3 WHERE d_fk=2

-- Enable FK Constraint
ALTER TABLE emp WITH CHECK CHECK CONSTRAINT fk_dept_d_fk

-- Following SELECTS will show the new values in updated records:
SELECT * from dept

d_id	dept
1	finance
3	hr	-- Updated record

SELECT * from emp

sn	ename	d_fk
1	Manoj	1
2	Saurabh	3	-- Updated record
3	Kanchan	1
4	Pooja	3	-- Updated record

-- Final Cleanup, DROP the tables.
DROP TABLE emp
DROP TABLE dept

MSDN BOL: http://msdn.microsoft.com/en-us/library/10cetyt6(v=VS.80).aspx