I today ran into a really weird problem in SQL Server (both 2008R2 and 2012). I'm trying to build up a string using concatenation in combination with a select statement.
I have found workarounds, but I would really like to understand what's going on here and why it doesn't give me my expected result. Can someone explain it to me?
http://sqlfiddle.com/#!6/7438a/1
On request, also the code here:
-- base table
create table bla (
[id] int identity(1,1) primary key,
[priority] int,
[msg] nvarchar(max),
[autofix] bit
)
-- table without primary key on id column
create table bla2 (
[id] int identity(1,1),
[priority] int,
[msg] nvarchar(max),
[autofix] bit
)
-- table with nvarchar(1000) instead of max
create table bla3 (
[id] int identity(1,1) primary key,
[priority] int,
[msg] nvarchar(1000),
[autofix] bit
)
-- fill the three tables with the same values
insert into bla ([priority], [msg], [autofix])
values (1, 'A', 0),
(2, 'B', 0)
insert into bla2 ([priority], [msg], [autofix])
values (1, 'A', 0),
(2, 'B', 0)
insert into bla3 ([priority], [msg], [autofix])
values (1, 'A', 0),
(2, 'B', 0)
;
declare @a nvarchar(max) = ''
declare @b nvarchar(max) = ''
declare @c nvarchar(max) = ''
declare @d nvarchar(max) = ''
declare @e nvarchar(max) = ''
declare @f nvarchar(max) = ''
-- I expect this to work and generate 'AB', but it doesn't
select @a = @a + [msg]
from bla
where autofix = 0
order by [priority] asc
-- this DOES work: convert nvarchar(4000)
select @b = @b + convert(nvarchar(4000),[msg])
from bla
where autofix = 0
order by [priority] asc
-- this DOES work: without WHERE clause
select @c = @c + [msg]
from bla
--where autofix = 0
order by [priority] asc
-- this DOES work: without the order by
select @d = @d + [msg]
from bla
where autofix = 0
--order by [priority] asc
-- this DOES work: from bla2, so without the primary key on id
select @e = @e + [msg]
from bla2
where autofix = 0
order by [priority] asc
-- this DOES work: from bla3, so with msg nvarchar(1000) instead of nvarchar(max)
select @f = @f + [msg]
from bla3
where autofix = 0
order by [priority] asc
select @a as a, @b as b, @c as c, @d as d, @e as e, @f as f
