SQL Server 字符串连接与 Varchar(max)

Question

我正在尝试通过将字符串连接成 varchar max 来生成列名列表，以用于动态查询。我一直使用的方法一直运行良好，直到我遇到一个超过 500 列的表。对于大表，当我将额外的字符串添加到列以生成 sql 选择脚本时，我的列列表被截断。 如果有人能向我指出为什么我的最后 4 个选择（如下）并非全部产生 600 行（当该字段被复制到新的查询窗口时，该字段包含换行符），我将非常感激。 Varchar(max) 应该足够大以产生我想要的结果。我只是无法弄清楚为什么会发生截断。

--CREATE TABLE [dbo].[Fact_NULLs_TEST]
--(
--My_really_quite_quite_quite_long_Column_name1 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name2 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name3 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name4 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name5 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name6 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name7 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name8 DECIMAL(18,2) NULL,
--My_really_quite_quite_quite_long_Column_name9 DECIMAL(18,2) NULL,
---- ......... etc up to My_really_quite_quite_quite_long_Column_name600
--)

DECLARE @Str AS VARCHAR(MAX)
DECLARE @Str2 AS VARCHAR(MAX)
DECLARE @Str3 AS VARCHAR(MAX)
DECLARE @Str4 AS VARCHAR(MAX)
SET @Str = ''
SET @Str2 = ''
SET @Str3 = ''
SET @Str4 = ''


SELECT 
    @Str = @Str + CAST(QUOTENAME(name) AS VARCHAR(MAX)) +  CAST(CHAR(13) AS VARCHAR(MAX))
FROM
MY_DB.sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')


SELECT 
    @Str2 = CAST(@Str2 AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) + CAST('my short text' AS VARCHAR(MAX)) + CAST(CHAR(13) AS VARCHAR(MAX))
FROM
MY_DB.sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')


SELECT 
    @Str3 = CAST(@Str3 AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) + CAST(' my long text my long text my long text my long text my long text ' AS VARCHAR(MAX)) + CAST(CHAR(13) AS VARCHAR(MAX))
FROM
MY_DB.sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')



SELECT 
    @Str4 = CAST(@Str4 AS VARCHAR(MAX)) + CAST('SUM(CASE WHEN ' AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) +  CAST(' IS NULL THEN 1 ELSE 0 END) AS ' AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) +  CAST(',' AS VARCHAR(MAX)) +  CAST(CHAR(13) AS VARCHAR(MAX))
FROM
MY_DB.sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')



SELECT @Str , LEN(@Str) --- 600 rows

SELECT @Str2, LEN(@Str2) --- 600 rows

SELECT @Str3 , LEN(@Str3) --- 378 rows ??????????????????

SELECT @Str4 , LEN(@Str4) --- 303 rows ??????????????????

Answer 1

如果输入字符串超过其最大值（128 个字符），则会发生这种情况，因为 quotename 将返回 null。 更多信息：

https://docs.microsoft.com/en-us/sql/t-sql/functions/quotename-transact-sql?view=sql-server-2017

在您的代码中也不需要在每个部分上添加强制转换，因为您正在连接声明的 varchar(max) 变量， Str 您可以简单地使用加号添加所有内容，甚至是括号，如下所示：

@Str = @Str + '['+name+']'+ CHAR(13)

无论如何，对于从单个可选对象返回的最大长度似乎没有记录的限制，或者使用 SQLCMD 并使用 SQL Server Management Studio 作为客户端执行 SQL，而不是将所有内容放在一个单一的字符串，您可以通过简单的直接选择获得正确的文本，或者如果您在程序中使用它，您需要打开一个游标并分别从结果集中获取每一行。

我包含了我用来测试的确切 SQL，并带有注释我的发现。

USE MY_DB

DECLARE @Str AS VARCHAR(MAX)
DECLARE @Str2 AS VARCHAR(MAX)
DECLARE @Str3 AS VARCHAR(MAX)
DECLARE @Str4 AS VARCHAR(MAX)
SET @Str = ''
SET @Str2 = ''
SET @Str3 = ''
SET @Str4 = ''


SELECT  
    @Str = @Str + '[' + name + ']'+ CHAR(13) 
FROM
sys.columns 
WHERE [object_id] = OBJECT_ID('[dbo].Fact_NULLs_TEST')

/*
SELECT 
    @Str2 = CAST(@Str2 AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) + CAST('my short text' AS VARCHAR(MAX)) + CAST(CHAR(13) AS VARCHAR(MAX))
FROM
sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')


SELECT 
    @Str3 = CAST(@Str3 AS VARCHAR(MAX)) +  CAST(QUOTENAME(name) AS VARCHAR(MAX)) + CAST(' my long text my long text my long text my long text my long text ' AS VARCHAR(MAX)) + CAST(CHAR(13) AS VARCHAR(MAX))
FROM
sys.columns 
WHERE [object_id] = OBJECT_ID('[MY_DB].[dbo].Fact_NULLs_TEST')


*/
SELECT 
    --top 50 
    @Str4 = @Str4 + 
    ' SUM(CASE WHEN ' +'['+ sc.name +']' + ' IS NULL THEN 1 ELSE 0 END) AS ' +'[' + sc.name +'],' + CHAR(13)
FROM
sys.columns sc
WHERE [object_id] = OBJECT_ID('[dbo].Fact_NULLs_TEST')

SELECT @Str , LEN(@Str) --- 29892 chars
/*
SELECT @Str2, LEN(@Str2) --- 

SELECT @Str3 , LEN(@Str3) ---
*/
SELECT @Str4 , LEN(@Str4) --- 87384 chars BUT in case you use Microsoft SQL Server Management Studio and copy /paste into an editor this is coming out truncated

PRINT @Str4 --This is truncated as well if displayed from Messages tab within Microsoft SQL Server Management Studio or grabbed by using SQLCMD 
--with -o on a text file, i.e. SQLCMD -S dbserver -E -i c:\temp\test1.sql -o c:\temp\test1out.txt

SELECT 
    ' SUM(CASE WHEN ' +'['+ sc.name +']' + ' IS NULL THEN 1 ELSE 0 END) AS ' +'[' + sc.name +'],' 
FROM
sys.columns sc
WHERE [object_id] = OBJECT_ID('[dbo].Fact_NULLs_TEST') 
--This last select works fine either from Microsoft SQL Server Management Studio or by using SQLCMD