Unexpected result in SELECT CASE WHEN NULL

Question

Schema and data

I have two tables with the following schema and data:

#table1:

create table #table1(
    PK int IDENTITY(1,1) NOT NULL, 
    [TEXT] nvarchar(50) NOT NULL
);

PK    TEXT
1     a
2     b
3     c
4     d
5     e

#table2:

create table #table2(
    PK int IDENTITY(1,1) NOT NULL, 
    FK int NOT NULL, 
    [TEXT] nvarchar(50) NOT NULL
);

PK    FK    TEXT
1     2     B
2     3     C

Problem

Now, if I select all from #table1 and left join #table2 like this:

select 
    #table1.PK,
    (case #table2.[TEXT] when NULL then #table1.[TEXT] else #table2.[TEXT] end) as [TEXT]
from 
    #table1
left join
    #table2 on #table2.FK = #table1.PK
;

the output are as following:

PK    TEXT
1     NULL
2     B
3     C
4     NULL
5     NULL

Question

I expected the result to be:

PK    TEXT
1     a     <
2     B
3     C
4     d     <
5     e     <

So why does this happen (or what am I doing wrong) and how can I fix this?

Source code

if (OBJECT_ID('tempdb..#table1') is not null) drop table #table1;
if (OBJECT_ID('tempdb..#table2') is not null) drop table #table2;

create table #table1(PK int IDENTITY(1,1) NOT NULL, [TEXT] nvarchar(50) NOT NULL);
create table #table2(PK int IDENTITY(1,1) NOT NULL, FK int NOT NULL, [TEXT] nvarchar(50) NOT NULL);

insert into #table1 ([TEXT]) VALUES ('a'), ('b'), ('c'), ('d'), ('e');
insert into #table2 (FK, [TEXT]) VALUES (2, 'B'), (3, 'C');

select 
    #table1.PK,
    (case #table2.[TEXT] when NULL then #table1.[TEXT] else #table2.[TEXT] end) as [TEXT]
from 
    #table1
left join
    #table2 on #table2.FK = #table1.PK
;

drop table #table1;
drop table #table2;

Answer 1

From my perspective this is equivalent of

select isnull(table2.text, table1.text) as text from ...

Answer 2

You should check whether a field is null or not by is null, even though your case when is syntactically correct, you should use the other syntactically correct version.

case
  when #table2.[TEXT] is null then #table1.[TEXT]
  else #table2.[TEXT]
end

Answer 3

The problem is the way you have constructed your CASE statement. Any CASE statement of the form CASE x WHEN NULL THEN... is not going to behave as you might initially expect as you are effectively performing a comparison with NULL, which is always false, in your case resulting in always getting #table2.[TEXT].

I think you'd need to do:

(CASE WHEN #table2.[TEXT] IS NULL THEN #table1.[TEXT] ELSE #table2.[TEXT] END) AS [TEXT]

which is equivalent to COALESCE:

COALESCE(#table2.[TEXT], #table1.[TEXT]) AS [TEXT]