我今天在 SQL Server（2008R2 和 2012）中遇到了一个非常奇怪的问题。我正在尝试使用连接结合select语句来构建字符串。
我找到了解决方法，但我真的很想了解这里发生了什么以及为什么它没有给我预期的结果。有人可以向我解释吗？
http://sqlfiddle.com/#!6/7438a/1
根据要求，还有这里的代码：
-- 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