When working with SQL Server, you may come across scenarios where you need to enforce a constraint or rule on a table, but you don’t want to validate the existing data. This can be useful in situations where you want to ensure data integrity going forward without having to correct the data that already exists in the table.

In Oracle, you can achieve this by using the NOVALIDATE syntax. However, in SQL Server, the equivalent syntax is NOCHECK. Let’s take a look at an example to understand this concept better.

First, let’s create a table and populate it with some records:

CREATE TABLE tblIdentity
(
ID INT IDENTITY(1,1),
Cols VARCHAR(100)
)

INSERT INTO tblIdentity (Cols)
VALUES ('RandomValues')
GO 10

Now, let’s try to create a constraint on the ID column:

ALTER TABLE tblIdentity
ADD CONSTRAINT CHECKID
CHECK (ID > 5)
GO

When running the above script, you will encounter an error because the table already contains values that do not satisfy the constraint.

To overcome this issue, you can create a constraint with the WITH NOCHECK keyword, which will skip the validation of existing values in the table. This syntax is similar to the NOVALIDATE script in Oracle.

ALTER TABLE tblIdentity
WITH NOCHECK
ADD CONSTRAINT CHECKID
CHECK (ID > 5)
GO

By running the script above, the constraint will be added to the table without validating the previous values. Any new inserts that violate the constraint will be prevented.

Using the NOCHECK syntax allows you to enforce constraints going forward without the need to correct existing data. This can be particularly useful in scenarios where you want to maintain data integrity while minimizing the impact on existing records.

Remember to use this feature judiciously and ensure that it aligns with your specific business requirements.