SQL Server: The ALTER TABLE statement conflicted with the CHECK constraint - sql-server

Question

I am confused about why this doesn't work:
BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
ALTER TABLE dbo.Accounts ADD CONSTRAINT
CK_client_not_null CHECK (site <> 'LWOP' and client is not null)
GO
ALTER TABLE dbo.Accounts SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
will throw this error:
Msg 547, Level 16, State 0, Line 13
The ALTER TABLE statement conflicted with the CHECK constraint "CK_client_not_null". The conflict occurred in database "ss", table "dbo.Accounts".
Msg 3902, Level 16, State 1, Line 18
The COMMIT TRANSACTION request has no corresponding BEGIN TRANSACTION.
but when I run the following:
select *
from Accounts
where site = 'LWOP' or client is null
I am getting only that row that can have client as null.
What am I missing here?
Once again: I want to make a constraint that will restrict null values on client column ONLY if site <> 'LWOP', so how to do it?
Thanks,
Dejan

Answer 1

I found the solution already:
BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
ALTER TABLE dbo.Accounts ADD CONSTRAINT
CK_client_not_null CHECK (site <> 'LWOP' and client is not null OR site = 'LWOP')
GO
ALTER TABLE dbo.Accounts SET (LOCK_ESCALATION = TABLE)
GO
COMMIT
site <> 'LWOP' and client is not null OR site = 'LWOP' - that covers all situations for check to work.