What's the difference between WITH CHECK ADD CONSTRAINT followed by CHECK CONSTRAINT and just ADD CONSTRAINT in SQL Server?

Question

I'm looking at the AdventureWorks sample database for SQL Server 2008, and I see in their creation scripts that they tend to use the following:

ALTER TABLE [Production].[ProductCostHistory] WITH CHECK ADD 
CONSTRAINT [FK_ProductCostHistory_Product_ProductID] FOREIGN KEY([ProductID])
  REFERENCES [Production].[Product] ([ProductID])
GO

followed immediately by :

ALTER TABLE [Production].[ProductCostHistory] CHECK CONSTRAINT     
[FK_ProductCostHistory_Product_ProductID]
GO

I see this for foreign keys (as here), unique constraints and regular CHECK constraints; DEFAULT constraints use the regular format I am more familiar with such as:

ALTER TABLE [Production].[ProductCostHistory] ADD  CONSTRAINT  
[DF_ProductCostHistory_ModifiedDate]  DEFAULT (getdate()) FOR [ModifiedDate]
GO

What is the difference, if any, between doing it the first way versus the second?

Answer 1

The first syntax is redundant - the WITH CHECK is default, and the constraint is turned on by default as well. This syntax is generated by the SQL management studio when generating sql scripts -- I'm assuming it's some sort of extra redundancy, possibly to ensure the constraint is enabled even if the default constraint behavior for a table is changed.

Answer 2

Hi,

WITH CHECK is indeed the default behaviour however it is good practice to include within your coding.

The alternative behaviour is of course to use WITH NOCHECK, so it is good to explicitly define your intentions. This is often used when you are playing with/modifying/switching inline partitions.

Cheers, John

Answer 3

Foreign key and check constraints have the concept of being trusted or untrusted, as well as being enabled and disabled. See the MSDN page for ALTER TABLE for full details.

WITH CHECK is the default for adding new foreign key and check constraints, WITH NOCHECK is the default for re-enabling disabled foreign key and check constraints. It's important to be aware of the difference.

Having said that, any apparently redundant statements generated by utilities are simply there for safety and/or ease of coding. Don't worry about them.

Answer 4

WITH NOCHECK is used as well when one has existing data in a table that doesn't conform to the constraint as defined and you don't want it to run afoul of the new constraint that you're implementing...