T-SQL是否支持插值字符串？
我们举个例子：
SET @query = 'SELECT ' + @somevariable + ' FROM SOME_TABLE'
我希望能够做到这样的事情：
SET @query = 'SELECT {@somevariable} FROM SOME_TABLE'
感谢您的回答！
答案 0 :(得分：5)
这不是在T-SQL中做事的常用方法，但可以使用xp_sprintf
DECLARE @query AS VARCHAR(100)
,@somevariable as VARCHAR(10) = '[id]'
EXEC xp_sprintf @query OUTPUT, 'SELECT %s FROM SOME_TABLE', @somevariable
PRINT @query
答案 1 :(得分：2)
给@ j.f.sebastian提供学分以指出这些解决方案。遗憾的是，xp_sprintf限制为254个字符，因此在使用长查询时这并不理想。而FORMATMESSAGE仅限于2047个字符，因此足以运行长查询。
我将在一篇文章中总结解决方案的所有内容，以保持组织有序。
使用FORMATMESSAGE重要的是要知道，使用插值字符串作为参数的第一个，它仅支持SQL版本2012及更高版本，因此我将使用FORMATMESSAGE发布2个答案：
SQL版本＆gt; = 2012：
SET @query = FORMATMESSAGE('SELECT %s FROM SOME_TABLE', @somevariable);
SQL版本＆lt; 2012：
EXEC sp_addmessage 50001, 16, 'SELECT %s FROM SOME_TABLE', NULL, NULL, 'replace'
SET @query = FORMATMESSAGE(50001, @somevariable)
使用xp_sprintf存储过程非常重要，因为它限制为254个字符，因此长时间查询不是一个好主意。
DECLARE @query AS VARCHAR(100)
,@somevariable as VARCHAR(10) = '[id]'
EXEC xp_sprintf @query OUTPUT, 'SELECT %s FROM SOME_TABLE', @somevariable
答案 2 :(得分：0)
我喜欢对动态SQL执行此操作，因此我wrote a function for SQL Server 2017+（使用STRING_AGG和JSON_VALUE）。可以对其进行重写以使其与旧版本兼容。
以下是链接消失的文字：
CREATE OR ALTER FUNCTION [Tools].[StringInterpolation]
(@Template VARCHAR(MAX)
, @JSON_Row NVARCHAR(MAX))
/*
This function replaces a string template with actual values from a JSON-formatted row
The table returns a single column: FormattedString
** Requires SQL Server 2017+ for STRING_AGG (could be rewritten using XML PATH)
** Requires SQL Server 2016+ for JSON_VALUE (maybe you could use XML)
EXAMPLE:
SELECT *
FROM (SELECT [Name] = 'Steven', Adjective = 'internet person', Verb = 'writes helpful(?) SQL functions') [d]
CROSS APPLY Tools.StringInterpolation ('{Name} is a {Adjective} who {Verb}.', (SELECT [d].* FOR JSON PATH))
Name | Adjective | Verb | FormattedString
-------+------------------------+---------------------------------+-----------------------------------------------------------------
Steven | internet person | writes helpful(?) SQL functions | Steven is a internet person who writes helpful(?) SQL functions.
*/
RETURNS TABLE
RETURN
WITH [CTE_10]
AS (SELECT [Number]
FROM(VALUES (1), (1), (1), (1), (1), (1), (1), (1), (1), (1) ) [v]([Number])),
-------------------
/* 100 rows (all 1s) */
[CTE_100]
AS (SELECT [Number] = 1
FROM [CTE_10] [a]
CROSS JOIN [CTE_10] [b]),
-------------------
/* 1,000,000 rows max (all 1s) */
[CTE_1000000]
AS (SELECT [Number] = 1
FROM [CTE_100] [a]
CROSS JOIN [CTE_100] [b]
CROSS JOIN [CTE_100] [c]),
-------------------
/* Numbers "Table" CTE: 1) TOP has variable parameter = DATALENGTH(@Template), 2) Use ROW_NUMBER */
[CTE_Numbers]
AS (SELECT TOP (ISNULL(DATALENGTH(@Template), 0))
[Number] = ROW_NUMBER() OVER(ORDER BY (SELECT NULL) )
FROM [CTE_1000000]),
-------------------
/* This is tricky. Get each start of each variable or non-variable
Variables look like {...}
Non-variables look like }...{ (i.e. the bits between the variables) */
[CTE_Start]
AS (SELECT [Type] = 'Text'
, [Start] = 1
UNION ALL
SELECT [Type] = IIF([Char] = '{', 'Variable', 'Text')
, [Start] = [Number] + 1 -- start *after* the { or }
FROM [CTE_Numbers]
CROSS APPLY (SELECT [Char] = SUBSTRING(@Template, [Number], 1)) [c]
WHERE [Char] IN ( '{', '}' ) ),
-------------------
/* Pair each "start" with the next to find indicies of each substring */
[CTE_StringIndicies]
AS (SELECT [Type]
, [Start]
, [End] = ISNULL(LEAD([Start]) OVER(
ORDER BY [Start]) - 1, DATALENGTH(@Template) + 1)
FROM [CTE_Start]),
-------------------
/* Get each substring */
[CTE_Variables]
AS (SELECT [Start]
, [Type]
, [SubString] = SUBSTRING(@Template, [Start], [End] - [Start])
FROM [CTE_StringIndicies]),
-------------------
/* If it's a variable, replace it with the actual value from @JSON_Row
Otherwise, just return the original substring */
[CTE_Replacements]
AS (SELECT [Start]
, [Substring] = IIF([Type] = 'Variable', JSON_VALUE(@JSON_Row, '$[0].' + [Substring]), [Substring])
FROM [CTE_Variables])
-------------------
/* Glue it all back together */
SELECT [FormattedString] = STRING_AGG([Substring], '') WITHIN GROUP (ORDER BY [Start])
FROM [CTE_Replacements];
答案 3 :(得分：0)
当你的字符串很长时，为了可读性，我喜欢这样做：
SET @query = replace( replace( replace(
'SELECT {@variable1} FROM {@variable2} WHERE {@variable3}'
, '{@variable1}', @variable1 )
, '{@variable2}', @variable2 )
, '{@variable3}', @variable3 )
它具有 SQL 字符串可读性的优点，而且您可以多次使用同一个变量。显然这不是正确的插值，但我比 FORMATMESSAGE() 选项更喜欢它，因为
答案 4 :(得分：0)
我喜欢的另一个解决方案来自 Stefan Hoffman 通过 this article
DECLARE @stmt_with_placeholders NVARCHAR(MAX) = 'USE {database}; SELECT * FROM {schema}.{table};'
DECLARE @stmt_processed NVARCHAR(MAX) = @stmt_with_placeholders
DECLARE @placeholders TABLE (
placeholder NVARCHAR(20)
,replacement_text NVARCHAR(200)
)
INSERT INTO @placeholders
VALUES
('{database}', 'WideWorldImporters'),
('{schema}', 'Warehouse'),
('{table}', 'PackageTypes')
SELECT
@stmt_processed = REPLACE(@stmt_processed,
placeholders.placeholder,
placeholders.replacement_text)
FROM
@placeholders as placeholders
SELECT @stmt_processed
EXEC (@stmt_processed)
答案 5 :(得分：0)
编写了 2 个方便的基于 Json 的插值函数。一个使用字典样式的 Json（键值对），另一个使用包含每个替换属性的 Json 对象。
1) 数据作为 Json 对象
例如，给定模板：'Hey, {name} is {age}'，运行：
SELECT [res4] = [ccgen].[fn_string_interpolation_object]('Hi, {name} is {age}' ,'{"name":"Alice", "age":24}')
... 返回 Hey, Alice is 24
CREATE OR ALTER FUNCTION [CcGen].[fn_string_interpolation_object](
@template NVARCHAR(4000), @data_json NVARCHAR(4000)) RETURNS NVARCHAR(4000) AS
/*
=============================================
C# or Java like string interpolation brought to TSQL.
example - copy to run proc
-----------------------
--property names must match those in template. Same as a dynamic object in C#
SELECT [res4] = [ccgen].[fn_string_interpolation_object]('Hi, {name} is {age}' ,'{"name":"Alice", "age":24}')
-- returns
Hi, Alic is 24
=============================================
*/
BEGIN
SELECT @template = REPLACE(@template ,'{' + [key] + '}' ,[value]) FROM OPENJSON(@data_json);
RETURN @template;
END;
2) 数据为 Json 字典
例如，给定模板：'Hey, {name} is {age}'，运行：
SELECT [res2] = [ccgen].[fn_string_interpolation]('Hey, {name} is {age}','{"items":[{"key":"name", "value":"Alice"},{"key":"age", "value":"24"}]}')
... 返回 Hey, Alice is 24
CREATE OR ALTER FUNCTION [CcGen].[fn_string_interpolation](
@template NVARCHAR(4000),
@key_value_json NVARCHAR(4000)) RETURNS NVARCHAR(4000) AS
/*
=============================================
C# or Java like string interpolation brought to TSQL.
example - copy to run proc
-----------------------
DECLARE @json NVARCHAR(2048) = N'{
"items": [
{
"key": "A",
"value": "a1"
},
{
"key": "B",
"value": "b2"
},
{
"key": "C",
"value": "c3"
}
]
}';
DECLARE @template NVARCHAR(4000) = 'message:= A:{A}, B:{B}, C:{C}'
select res = ccgen.fn_string_interpolation(@template, @json)
-- returns
formatted 3 = A:a1, B:b2, C:c3
=============================================
*/
BEGIN
SELECT @template = REPLACE(@template ,'{' + [key] + '}' ,[value]) FROM OPENJSON(@key_value_json ,'$.items') WITH ( [key] VARCHAR(200) '$.key', [value] VARCHAR(4000) '$.value' );
RETURN @template;
END;
提示：Json 需要更多的输入。修改代码并将属性名称缩短为 "k" (key) 、"v" (value) 和 "d" (items) 以使其变小。然后调用更整洁：
SELECT [res2] = [ccgen].[fn_string_interpolation]('Hey, {name} is {age}','{"d":[{"k":"name", "v":"Alice"},{"k":"age", "v":"24"}]}')
注意事项：
当然，这仅适用于支持 Json 的 MSSQL 版本。也不支持转义。例如，不可能将 {a} 同时用作文字和替换参数。这可以使用转义添加，但不能保证努力，因为我没有使用该功能。