使用 Order By 时 COALESCE 的奇怪 TSQL 行为

Question

我在使用 coalesce 时遇到了一些非常奇怪的行为。当我没有指定 return 金额（TOP (50)）时，我只会得到一个最后的结果，但是如果我删除 "Order By" 它会起作用......下面的例子

DECLARE @result varchar(MAX)
SELECT @result = COALESCE(@result + ',', '') + [Title]
FROM Episodes WHERE [SeriesID] = '1480684' AND [Season] = '1' Order by [Episode] ASC
SELECT @result

只会return一个最后的结果：

The Shiva Bowl

但如果我指定最大 return 金额（仅将 TOP (50) 添加到同一语句）

DECLARE @result varchar(MAX)
SELECT TOP(50) @result = COALESCE(@result + ',', '') + [Title]
FROM Episodes WHERE [SeriesID] = '1480684' AND [Season] = '1' Order by [Episode] ASC
SELECT @result

我按正确顺序得到所有结果

The Draft,The Bounce Test,Sunday at Ruxin's,Mr. McGibblets,The Usual Bet,The Shiva Bowl

罪魁祸首似乎是 [Title] 列，就好像我 return 一个不同的列一样，它似乎在没有指定 return 限制的情况下工作。仅供参考 [Title] 是 VARCHAR(MAX) NOT NULL 列。

知道是什么原因造成的吗？我真的不想设置限制，但这是目前 return 处理所有数据的唯一方法...谢谢

Answer 1

您不能像这样依赖串联：

SELECT @result = COALESCE(@result + ',', '') + [Title]
FROM Episodes
...

Execution Plan and Results of Aggregate Concatenation Queries Depend Upon Expression Location

示例取决于 CTE/temp table/execution 计划你会得到不同的结果：

SqlFiddleDemo

DECLARE @text VARCHAR(MAX) = ''
       ,@text2 VARCHAR(MAX) = '';

SELECT CAST(ROW_NUMBER() OVER (ORDER BY name) AS INT) AS number 
INTO #numbers 
FROM master..spt_values 


;WITH numbers (number)
AS
(
    SELECT CAST(ROW_NUMBER() OVER (ORDER BY name) AS INT) AS number
    FROM master..spt_values 
),a AS
(
    SELECT number FROM numbers WHERE number < 10
)
SELECT      @text = @text + LTRIM(STR(a.number))
FROM        a
ORDER BY    a.number DESC


;WITH numbers (number)
AS
(
    SELECT number FROM #numbers
),
a
AS
(
    SELECT number FROM numbers WHERE number < 10
)
SELECT      @text2 = @text2 + LTRIM(STR(a.number))
FROM        a
ORDER BY    a.number DESC

SELECT @text, @text2;

我制作了示例，您的第一个查询有效 SqlFiddleDemo。但是您的解决方案高度依赖于执行计划。

改为使用 XML + STUFF 进行串联。

使用 Order By 时 COALESCE 的奇怪 TSQL 行为

Strange TSQL behavior with COALESCE when using Order By

tsql

sql-server-2012

sql-order-by

coalesce