Question

I have a script to update a database by checking for a foreign key’s existence and, if it doesn’t exist, creating it. It was generated by SQL Management Studio.

IF NOT EXISTS (SELECT * FROM sys.foreign_keys WHERE object_id = OBJECT_ID(N'[dbo].[FK_tblChangeRequestForecast_tblCostingCenter]') AND parent_object_id = OBJECT_ID(N'[dbo].[tblChangeRequestForecast]'))
ALTER TABLE [dbo].[tblChangeRequestForecast]  WITH CHECK ADD  CONSTRAINT [FK_tblChangeRequestForecast_tblCostingCenter] FOREIGN KEY([CostingCenterID])
REFERENCES [dbo].[tblCostingCenter] ([CostingCenterID])
GO

IF  EXISTS (SELECT * FROM sys.foreign_keys WHERE object_id = OBJECT_ID(N'[dbo].[FK_tblChangeRequestForecast_tblCostingCenter]') AND parent_object_id = OBJECT_ID(N'[dbo].[tblChangeRequestForecast]'))
ALTER TABLE [dbo].[tblChangeRequestForecast] CHECK CONSTRAINT [FK_tblChangeRequestForecast_tblCostingCenter]
GO

The script raises an error when it’s run:

The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_tblChangeRequestForecast_tblCostingCenter". The conflict occurred in database "mydatabase", table "dbo.tblCostingCenter", column 'CostingCenterID'.

This is very mysterious. There is no trace of the foreign key as far as the queries can tell, but the create script fails with the above error. The server is running SQL Server 2005 SP3 (9.00.4035.00).

[Update]: I’ve just reproduced the issue on a SQL Server 2012 instance. So the version doesn’t appear to be too important.

Any idea what could cause this?

Answer 1

It is complaining about the FK constraint that you are trying to add because there is existing data in the table that would no longer be valid if the constraint was successfully applied.

Probably means there is one or more ChangeRequestForecast records where CostingCenterID has a value that doesn’t correspond with CostingCenter.CostingCenterID.