Is there a way to make a TSQL variable constant?

Question

Is there a way to make a TSQL variable constant?

Answer 1

My workaround to missing constans is to give hints about the value to the optimizer.

DECLARE @Constant INT = 123;

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

This tells the query compiler to treat the variable as if it was a constant when creating the execution plan. The down side is that you have to define the value twice.

Answer 2

The best answer is from SQLMenace according to the requirement if that is to create a temporary constant for use within scripts, i.e. across multiple GO statements/batches.

Just create the procedure in the tempdb then you have no impact on the target database.

One practical example of this is a database create script which writes a control value at the end of the script containing the logical schema version. At the top of the file are some comments with change history etc... But in practice most developers will forget to scroll down and update the schema version at the bottom of the file.

Using the above code allows a visible schema version constant to be defined at the top before the database script (copied from the generate scripts feature of SSMS) creates the database but used at the end. This is right in the face of the developer next to the change history and other comments, so they are very likely to update it.

For example:

use tempdb
go
create function dbo.MySchemaVersion()
returns int
as
begin
    return 123
end
go

use master
go

-- Big long database create script with multiple batches...
print 'Creating database schema version ' + CAST(tempdb.dbo.MySchemaVersion() as NVARCHAR) + '...'
go
-- ...
go
-- ...
go
use MyDatabase
go

-- Update schema version with constant at end (not normally possible as GO puts
-- local @variables out of scope)
insert MyConfigTable values ('SchemaVersion', tempdb.dbo.MySchemaVersion())
go

-- Clean-up
use tempdb
drop function MySchemaVersion
go

Answer 3

For enums or simple constants, a view with a single row has great performance and compile time checking / dependency tracking ( cause its a column name )

See Jared Ko's blog post https://blogs.msdn.microsoft.com/sql_server_appendix_z/2013/09/16/sql-server-variables-parameters-or-literals-or-constants/

create the view

 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]

use the view

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

Answer 4

There is no built-in support for constants in T-SQL. You could use SQLMenace's approach to simulate it (though you can never be sure whether someone else has overwritten the function to return something else…), or possibly write a table containing constants, as suggested over here. Perhaps write a trigger that rolls back any changes to the ConstantValue column?

Answer 5

Okay, lets see

Constants are immutable values which are known at compile time and do not change for the life of the program

that means you can never have a constant in SQL Server

declare @myvalue as int
set @myvalue = 5
set @myvalue = 10--oops we just changed it

the value just changed

Answer 6

No, but you can create a function and hardcode it in there and use that.

Here is an example:

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

SELECT dbo.fnConstant()