SQL Server 2019

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CREATE OR ALTER FUNCTION dbo.temp_fn_testvariable(@a int)
RETURNS TABLE
AS 
RETURN
(
    SELECT @a AS fnVal
    WHERE 1 <> 1
);

select *
  from dbo.temp_fn_testvariable(1) as ca ;

fnVal

select 1 as c where 1 <> 1 ;

c

select * 
  from (select 1 where 1 <> 1) AS ca(c) ;

c

select s.*, ca.* from (select 1 v) AS s 
outer apply dbo.temp_fn_testvariable(s.v) AS ca ;

v
fnVal

1
1

select s.*, ca.* from (select 1 v) AS s 
outer apply (select 1 where 1 <> 1) AS ca(c) ;

v
c

1
null