有没有办法让TSQL变量保持不变？

Question

有没有办法让TSQL变量保持不变？

Answer 1

不,但你可以在那里创建一个函数并对其进行硬编码并使用它.

这是一个例子:

CREATE FUNCTION fnConstant()
RETURNS INT
AS
BEGIN
    RETURN 2
END
GO

SELECT dbo.fnConstant()

Answer 2

我对缺少常量的解决方法是给优化器提供有关值的提示.

DECLARE @Constant INT = 123;

SELECT * 
FROM [some_relation] 
WHERE [some_attribute] = @Constant
OPTION( OPTIMIZE FOR (@Constant = 123))

这告诉查询编译器在创建执行计划时将变量视为常量.缺点是你必须定义两次值.

Answer 3

使用伪常量:http://blogs.msdn.com/b/sql_server_appendix_z/archive/2013/09/16/sql-server-variables-parameters-or-literals-or-constants.aspx

伪常数不是变量或参数.相反,它们只是一行的视图,以及足够的列来支持您的常量.使用这些简单的规则,SQL引擎完全忽略视图的值,但仍然根据其值构建执行计划.执行计划甚至没有显示视图的连接!

Answer 4

不,但应使用良好的旧命名约定.

declare @MY_VALUE as int

Answer 5

T-SQL中没有对常量的内置支持.您可以使用SQLMenace的方法来模拟它(尽管您永远无法确定其他人是否已覆盖该函数以返回其他内容...),或者可能会编写一个包含常量的表,如此处所示.也许写一个触发器来回滚ConstantValue列的任何更改？

Answer 6

在使用SQL函数之前,请运行以下脚本以查看性能差异:

IF OBJECT_ID('fnFalse') IS NOT NULL
DROP FUNCTION fnFalse
GO

IF OBJECT_ID('fnTrue') IS NOT NULL
DROP FUNCTION fnTrue
GO

CREATE FUNCTION fnTrue() RETURNS INT WITH SCHEMABINDING
AS
BEGIN
RETURN 1
END
GO

CREATE FUNCTION fnFalse() RETURNS INT WITH SCHEMABINDING
AS
BEGIN
RETURN ~ dbo.fnTrue()
END
GO

DECLARE @TimeStart DATETIME = GETDATE()
DECLARE @Count INT = 100000
WHILE @Count > 0 BEGIN
SET @Count -= 1

DECLARE @Value BIT
SELECT @Value = dbo.fnTrue()
IF @Value = 1
    SELECT @Value = dbo.fnFalse()
END
DECLARE @TimeEnd DATETIME = GETDATE()
PRINT CAST(DATEDIFF(ms, @TimeStart, @TimeEnd) AS VARCHAR) + ' elapsed, using function'
GO

DECLARE @TimeStart DATETIME = GETDATE()
DECLARE @Count INT = 100000
DECLARE @FALSE AS BIT = 0
DECLARE @TRUE AS BIT = ~ @FALSE

WHILE @Count > 0 BEGIN
SET @Count -= 1

DECLARE @Value BIT
SELECT @Value = @TRUE
IF @Value = 1
    SELECT @Value = @FALSE
END
DECLARE @TimeEnd DATETIME = GETDATE()
PRINT CAST(DATEDIFF(ms, @TimeStart, @TimeEnd) AS VARCHAR) + ' elapsed, using local variable'
GO

DECLARE @TimeStart DATETIME = GETDATE()
DECLARE @Count INT = 100000

WHILE @Count > 0 BEGIN
SET @Count -= 1

DECLARE @Value BIT
SELECT @Value = 1
IF @Value = 1
    SELECT @Value = 0
END
DECLARE @TimeEnd DATETIME = GETDATE()
PRINT CAST(DATEDIFF(ms, @TimeStart, @TimeEnd) AS VARCHAR) + ' elapsed, using hard coded values'
GO

Answer 7

如果您有兴趣为变量中的值获取最佳执行计划,则可以使用动态sql代码.它使变量保持不变.

DECLARE @var varchar(100) = 'some text'
DECLARE @sql varchar(MAX)
SET @sql = 'SELECT * FROM table WHERE col = '''+@var+''''
EXEC (@sql)

Answer 8

对于枚举或简单常量,具有单行的视图具有出色的性能和编译时检查/依赖性跟踪(导致其列名)

请参阅Jared Ko的博客文章https://blogs.msdn.microsoft.com/sql_server_appendix_z/2013/09/16/sql-server-variables-parameters-or-literals-or-constants/

创建视图

 CREATE VIEW ShipMethods AS
 SELECT CAST(1 AS INT) AS [XRQ - TRUCK GROUND]
   ,CAST(2 AS INT) AS [ZY - EXPRESS]
   ,CAST(3 AS INT) AS [OVERSEAS - DELUXE]
  , CAST(4 AS INT) AS [OVERNIGHT J-FAST]
   ,CAST(5 AS INT) AS [CARGO TRANSPORT 5]

使用视图

SELECT h.*
FROM Sales.SalesOrderHeader 
WHERE ShipMethodID = ( select [OVERNIGHT J-FAST] from ShipMethods  )