SQL Server 2017

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select @@version;


(No column name)


Microsoft SQL Server 2017 (RTM-CU30) (KB5013756) - 14.0.3451.2 (X64) 
	Jun 22 2022 18:20:15 
	Copyright (C) 2017 Microsoft Corporation
	Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)



CREATE TABLE SomeTable
(
   Id DECIMAL NOT NULL,
   UserIdentifier NVARCHAR(100) NULL UNIQUE,
   PRIMARY KEY (Id)
)
INSERT INTO SomeTable (Id, UserIdentifier) VALUES (1, 'ABC');
1 rows affected

INSERT INTO SomeTable (Id, UserIdentifier) VALUES (2, 'ABC');
Msg 2627 Level 14 State 1 Line 1
Violation of UNIQUE KEY constraint 'UQ__SomeTabl__463BA3014345A394'. Cannot insert duplicate key in object 'dbo.SomeTable'. The duplicate key value is (ABC).
Msg 3621 Level 0 State 0 Line 1
The statement has been terminated.