Possible to implement a manual increment with just simple SQL INSERT?

You understand that you will have collisions right?

you need to do something like this and this might cause deadlocks so be very sure what you are trying to accomplish here

DECLARE @id int
BEGIN TRAN

    SELECT @id = MAX(id) + 1 FROM Table1 WITH (UPDLOCK, HOLDLOCK)
    INSERT INTO Table1(id, data_field)
    VALUES (@id ,'[blob of data]')
COMMIT TRAN

To explain the collision thing, I have provided some code

first create this table and insert one row

CREATE TABLE Table1(id int primary key not null, data_field char(100))
GO
Insert Table1 values(1,'[blob of data]')
Go

Now open up two query windows and run this at the same time

declare @i int
set @i =1
while @i < 10000
begin
BEGIN TRAN

INSERT INTO Table1(id, data_field)
SELECT MAX(id) + 1, '[blob of data]' FROM Table1

COMMIT TRAN;
set @i =@i + 1
end

You will see a bunch of these

Server: Msg 2627, Level 14, State 1, Line 7 Violation of PRIMARY KEY constraint 'PK__Table1__3213E83F2962141D'. Cannot insert duplicate key in object 'dbo.Table1'. The statement has been terminated.

Try this instead:

INSERT INTO Table1 (id, data_field)
SELECT id, '[blob of data]' FROM (SELECT MAX(id) + 1 as id FROM Table1) tbl

I wouldn't recommend doing it that way for any number of reasons though (performance, transaction safety, etc)

It could be because there are no records so the sub query is returning NULL...try

INSERT INTO tblTest(RecordID, Text) 
VALUES ((SELECT ISNULL(MAX(RecordID), 0) + 1 FROM tblTest), 'asdf')