Returning 0 rows does not always mean NULL…

output

(1 row(s) affected)
Id          Name
----------- ----
1           asdf

(1 row(s) affected)

x
------------
untouched

(1 row(s) affected)

x
------------
NULL

(1 row(s) affected)

they_be_different.sql

DECLARE @x VARCHAR(12);
SELECT 1 Id, 'asdf' Name INTO #t;
SELECT * FROM #t;

SET @x = 'untouched';
SELECT TOP(1) @x = Name FROM #t WHERE Id = 0;
SELECT @x x;

SET @x = 'untouched';
SET @x = (SELECT TOP(1) Name FROM #t WHERE Id = 0);
SELECT @x x;

DROP TABLE #t;