Question

Does SQL Server allow constraint violations (i.e. deferred constraints) in a transaction as long as the transaction has not been committed yet?

I have a running, uncommitted transaction and while this transaction is running, I will change my data so that it will violate some constraints (like having duplicate primary keys for example). When I commit the transaction, the data will be in consistent, valid state. Is this generally allowed in SQL and specifically in MS SQL Server?

Answer 1

No, sorry. SQL Server does not allow deferred contraints in a transaction. It was present in SQL Server 6.5, but removed in SQL Server 2000:

SET DISABLE_DEF_CNST_CHK ON

Each individual statement must be consistent etc, regardless of whether it is in a transaction

Some RDBMS do allow this (e.g. Oracle, Postgres, Interbase)

Connect

There is a Microsoft Connect request, created in 2006, asking for this feature:

Option to defer foreign key constraint checking until transaction commit

There are various “chicken and egg” scenarios where it would be desirable to defer the checking of referential integrity constraints until commit time on a transaction.

Allow deferring of referential integrity constraint checking until commit time on a transaction (as an option). Suggest providing an option on BEGIN TRANSACTION that specifies this.

The last response from Microsoft came a decade ago:

Posted by Sameer [MSFT] on 10/13/2006 at 1:35 PM

Hello Greg,

Thanks for the feedback. We are aware of this and looking into it for a future release.

Sameer Verkhedkar
SQL Engine
[MSFT]

Which is Microsoft speak for “go away”.

SQL-92 defines it

The feature was defined in July 1992 with SQL-92. An example syntax would be:

BEGIN TRANSACTION
   SET CONSTRAINTS ALL DEFERRED --applies only to the current transaction

   INSERT Customers ...
   INSERT Orders ...
   UPDATE Customers ... --add the thing we were missing

COMMIT TRANSACTION