Why is this function non-deterministic? - sql-server

Question

Why does SQL Server think this function is non-deterministic?
CREATE FUNCTION [Util].[BuildStreetFullName]
(
#StreetNumber VARCHAR(20),
#StreetDir VARCHAR(2),
#StreetName VARCHAR(50) ,
#StreetType VARCHAR(4) ,
#StreetPostDir VARCHAR(2)
)
RETURNS VarChar(100)
AS
BEGIN
SET #StreetNumber = NULLIF( Util.Trim(#StreetNumber ), '');
SET #StreetDir = NULLIF( Util.Trim(#StreetDir ), '');
SET #StreetName = NULLIF( Util.Trim(#StreetName ), '');
SET #StreetType = NULLIF( Util.Trim(#StreetType ), '');
SET #StreetPostDir = NULLIF( Util.Trim(#StreetPostDir ), '');
DECLARE #Result VarChar(100) = #StreetNumber;
IF #StreetDir IS NOT NULL BEGIN
IF #Result IS NOT NULL
SET #Result = #Result + ' ' + #StreetDir;
ELSE
SET #Result = #StreetDir;
END
IF #StreetName IS NOT NULL BEGIN
IF #Result IS NOT NULL
SET #Result = #Result + ' ' + #StreetName;
ELSE
SET #Result = #StreetName;
END
IF #StreetType IS NOT NULL BEGIN
IF #Result IS NOT NULL
SET #Result = #Result + ' ' + #StreetType;
ELSE
SET #Result = #StreetType;
END
IF #StreetPostDir IS NOT NULL BEGIN
IF #Result IS NOT NULL
SET #Result = #Result + ' ' + #StreetPostDir;
ELSE
SET #Result = #StreetPostDir;
END
RETURN NULLIF(#Result, '');
END
CREATE FUNCTION [Util].[Trim]
(
#value nVarChar(max)
)
RETURNS nVarChar(max)
AS
BEGIN
RETURN LTrim(RTrim(#value))
END

Answer 1

1) You could create both function WITH SCHEMABINDING:
ALTER FUNCTION dbo.[Trim]
(
#value nVarChar(max)
)
RETURNS nVarChar(max)
WITH SCHEMABINDING
AS
...
and
ALTER FUNCTION dbo.[BuildStreetFullName]
(
#StreetNumber VARCHAR(20),
#StreetDir VARCHAR(2),
#StreetName VARCHAR(50) ,
#StreetType VARCHAR(4) ,
#StreetPostDir VARCHAR(2)
)
RETURNS VarChar(100)
WITH SCHEMABINDING
AS
...
This will force SQL Server to check if these function are deterministic or not.
SELECT OBJECTPROPERTY( OBJECT_ID(N'dbo.[BuildStreetFullName]') , 'IsDeterministic' ) AS IsDeterministic
Output:
IsDeterministic
---------------
1
2) Please don't use != NULL operator to check for NOT NULL. Use IS NOT NULL. At this moment ANSI_NULLS OFF is deprecated and a a future version of SQL Server will allow only ANSI_NULLS ON.
3) Scalar functions can be a nightmare from performance point of view. I would rewrite those functions as inline table valued functions.