動態SQL和spexecutesql行爲

Question

我有一個像

c1 c2 c3 c4 
----------------- 
2 1 7 13 
9 2 8 14 
1 3 9 15 
5 4 10 16 
2 5 11 17 
11 6 12 18 

的桌子，一般我不知道列數（代碼@d這裏4）得到一個字符串形式：

2,9,1,5,2,11, 1,2,3,4,5,6, 7,8,9,10,11,12, 13,14,15,16,17,18 

要做到這一點我做：

DECLARE @d INT 
     ,@counterI INT 
     ,@template AS nvarchar(max) 

    SET @d = 4;  
    SET @counterI = 1; 
    Set @template = 'SELECT STUFF( 
         ( SELECT '','' + CAST([col] AS VARCHAR) FROM (';       
    WHILE (@counterI < @d) BEGIN 
     SET @template += ' SELECT [c'+CAST(@counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] UNION ALL '; 
     SET @counterI = @counterI + 1; 
    END 
    Set @template += ' SELECT [c'+CAST(@counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] ' 
    Set @template += ') alldata FOR XML PATH('''') ) , 1 , 1 , '''')';  

declare @CommaString varchar(max) 
set @CommaString = '' 
exec sp_executesql @template, N'@CommaString varchar(max) out', @CommaString out 

所以，如果我做

select @CommaString; 

爲什麼不@CommaString在選擇它返回字符串的那一刻，如果這樣做的sp_executesql的當它打印出來嗎？

Answer 1

我可能會遺漏一些關於sp_executesql如何工作的內容，但是你不需要像@template中的'SELECT @CommaString = ...'這樣的東西，以便它將逗號字符串分配給out參數嗎？

只是爲了澄清，我想你需要的東西，如：

DECLARE @d INT 
     ,@counterI INT 
     ,@template AS nvarchar(max) 

SET @d = 4;  
SET @counterI = 1; 
Set @template = 'SELECT @CommaString = STUFF( 
        ( SELECT '','' + CAST([col] AS VARCHAR) FROM (';       
WHILE (@counterI < @d) BEGIN 
    SET @template += ' SELECT [c'+CAST(@counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] UNION ALL '; 
    SET @counterI = @counterI + 1; 
END 
Set @template += ' SELECT [c'+CAST(@counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] ' 
Set @template += ') alldata FOR XML PATH('''') ) , 1 , 1 , '''')';  

declare @CommaString varchar(max) 
set @CommaString = '' 
exec sp_executesql @template, N'@CommaString varchar(max) out', @CommaString = @CommaString out 

作爲一個簡單的例子，這樣的事情也許是更容易閱讀/明白我的意思：

declare @CommaString varchar(max) 
set @CommaString = '' 
exec sp_executesql 'SELECT @CommaString = ''1,2,3''', '@CommaString varchar(max) out', @CommaString = @CommaString out 

順便說一句，我通常看到這種類型的字符串連接：

DECLARE @MyString varchar(max) 

SET @MyString = '' 

SELECT @MyString = @MyString + ',' + MyColumn 
    FROM MyTable