Is there a way to make a TSQL variable constant?

Question

Is there a way to make a TSQL variable constant?

Answer 1

Since there is no build in support for constants, my solution is very simple.

Since this is not supported:

Declare Constant @supplement int = 240
SELECT price + @supplement
FROM   what_does_it_cost

I would simply convert it to

SELECT price + 240/*CONSTANT:supplement*/
FROM   what_does_it_cost

Obviously, this relies on the whole thing (the value without trailing space and the comment) to be unique. Changing it is possible with a global search and replace.

Answer 2

There are no such thing as "creating a constant" in database literature. Constants exist as they are and often called values. One can declare a variable and assign a value (constant) to it. From a scholastic view:

DECLARE @two INT
SET @two = 2

Here @two is a variable and 2 is a value/constant.

Answer 3

Prior to using a SQL function run the following script to see the differences in performance:

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 4

No, but good old naming conventions should be used.

declare @MY_VALUE as int

Answer 5

One solution, offered by Jared Ko is to use pseudo-constants.

As explained in SQL Server: Variables, Parameters or Literals? Or… Constants?:

Pseudo-Constants are not variables or parameters. Instead, they're simply views with one row, and enough columns to support your constants. With these simple rules, the SQL Engine completely ignores the value of the view but still builds an execution plan based on its value. The execution plan doesn't even show a join to the view!

Create like this:

CREATE SCHEMA ShipMethod
GO
-- Each view can only have one row.
-- Create one column for each desired constant.
-- Each column is restricted to a single value.
CREATE VIEW ShipMethod.ShipMethodID 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]

Then use like this:

SELECT h.*
FROM Sales.SalesOrderHeader h
JOIN ShipMethod.ShipMethodID const
    ON h.ShipMethodID = const.[OVERNIGHT J-FAST]

Or like this:

SELECT h.*
FROM Sales.SalesOrderHeader h
WHERE h.ShipMethodID = (SELECT TOP 1 [OVERNIGHT J-FAST] FROM ShipMethod.ShipMethodID)

Answer 6

If you are interested in getting optimal execution plan for a value in the variable you can use a dynamic sql code. It makes the variable constant.

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