Persisted column throwing non-deterministic error using convert - sql

Question

Using SQL Server 2016 and I'm running into a little bit of a problem.
Here's my use case that is causing issues for me...
create table dbo.Example (
Id int identity (1, 1) not null,
[Name] nvarchar(100) not null,
Email nvarchar(255) not null,
DOB datetime2(7) not null,
RowHash as convert(nvarchar(66), hashbytes('SHA1', coalesce(
convert(nvarchar(max), [Name]),
convert(nvarchar(max), Email),
convert(nvarchar(max), DOB)
))) persisted
constraint [PK_Example] primary key clustered (Id asc)
);
drop table dbo.Example;
The message I'm getting is:
Msg 4936, Level 16, State 1, Line 1
Computed column 'RowHash' in table 'Example' cannot be persisted because the column is non-deterministic.
When I set the column to not be persisted, the data type is interpreted correctly as nvarchar(66) however I would like to have it persisted. The issue seems to be related to the datetime2 column however I have a mixture of data types on the table.
So the goals are to use a persisted hashbytes column to hold a hash of all the values in my table.
Any ideas?
Thx!

Answer 1

Why coalesce() and not concat()?
Example
create table dbo.Example (
Id int identity (1, 1) not null,
[Name] nvarchar(100) not null,
Email nvarchar(255) not null,
DOB datetime2(7) not null,
RowHash as convert(nvarchar(66), hashbytes('SHA1', concat(
[Name],
Email,
DOB
))) persisted
constraint [PK_Example] primary key clustered (Id asc)
);
Select * from [dbo].[Example]
--drop table dbo.Example;
Results

Answer 2

You can fix this by specifying a format for the date conversion:
create table dbo.Example (
Id int identity (1, 1) not null,
[Name] nvarchar(100) not null,
Email nvarchar(255) not null,
DOB date not null, -- I figure date is good enough
RowHash as convert(nvarchar(66), hashbytes('SHA1', concat(
convert(nvarchar(max), [Name]),
convert(nvarchar(max), Email),
convert(nvarchar(max), DOB, 121)
))) persisted
constraint [PK_Example] primary key clustered (Id asc)
);
The problem is that the default date-to-string conversion depends on system parameters, so it is not deterministic. For a persisted column, all components need to be deterministic.
I would love to say that the documentation covers this exotic point in well-balanced detail. Not quite. You can get the idea from this documentation. Just be forgiving -- it also applies to date, datetime2 and other data types.

Answer 3

Here's the final result which is a combo of both answers above. Thx very much for the help.
create table dbo.Example (
Id int identity (1, 1) not null,
[Name] nvarchar(100) not null,
Email nvarchar(255) not null,
DOB datetime2(7) null,
RowHash as convert(nvarchar(66), hashbytes('SHA1', concat(
convert(nvarchar(max), [Name]),
convert(nvarchar(max), Email),
convert(nvarchar(max), DOB, 121)
))) persisted
constraint [PK_Example] primary key clustered (Id asc)
);
drop table dbo.Example;