Determine That This Is Deterministic

Hi all... This is the definition on the M/S site:

"Deterministic functions always return the same result any time they are called with a specific set of input values and given the same state of the database. Nondeterministic functions may return different results each time they are called with a specific set of input values even if the database state that they access remains the same."

Good... straight forward, right? Ok.... try entering thses command seperately:


Create Table Readings (ReadingDate DateTime Not Null);



Create Function [dbo].[funct_SameDate](@datReadingDate Datetime)

Returns DateTime

As

Begin

return @datReadingDate;

End


Alter table Readings add

[TempColumn] as (dbo.[funct_SameDate](ReadingDate)) PERSISTED NOT NULL;


Error on last command returns:

Computed column 'TempColumn' in table 'Readings' cannot be persisted because the column is non-deterministic.


Can someone please explain this to me? The same value is always being returned.


This does work:


Create Table Readings (ReadingDate DateTime Not Null);

Alter table Readings add [TempColumn] as (ReadingDate) PERSISTED NOT NULL;


I obviously want to do more things inside the function, but I can't get by the first step.

Any suggestions?

Thanks!

Forch