SQL Server 2012

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select @@version; 

(No column name)

Microsoft SQL Server 2012 (SP4) (KB4018073) - 11.0.7001.0 (X64) 
	Aug 15 2017 10:23:29 
	Copyright (c) Microsoft Corporation
	Express Edition (64-bit) on Windows NT 6.3 <X64> (Build 9600: ) (Hypervisor)

CREATE TABLE a (x INT, y INT);
INSERT INTO a (x) VALUES (2), (3), (4);
3 rows affected

SELECT * FROM a;

x
y

2
null

3
null

4
null

UPDATE a SET y = x
WHERE x <= 3;
2 rows affected

SELECT * FROM a;

x
y

2
2

3
3

4
null

x	y
2	null
3	null
4	null

x	y
2	2
3	3
4	null