SQL Identity(1,1) as a Default constrain - sql

Question

I'm trying to define a new table as follows and I want the IndexPosition column to get the MAX(IndexPosition) + 1 by default.
CREATE TABLE SpeechOutputList
(ID int NOT NULL IDENTITY(1,1),
IndexPosition int DEFAULT (???),
SpeechConfigCode nvarchar(36) NOT NULL,
OutputSentence nvarchar(500),
PRIMARY KEY(ID),
FOREIGN KEY(SpeechConfigCode) REFERENCES SpeechConfig ON UPDATE CASCADE ON DELETE CASCADE);
I want to allow the user to set it's own custom number, but if he hasn't supplied any such number, the default would be the MAX(IndexPosition) + 1.
I thought about DEFAULT IDENTITY(1,1) but it's impossible.
I also thought about: DEFAULT SELECT MAX(IndexPosition) + 1 FROM SpeechOutputList but it's impossible too (Error: 'Subqueries are not allowed in this context. Only scalar expressions are allowed.').
Anyone has an idea?

Answer 1

You can do it using a SEQUENCE object.
SEQUENCE objects are more flexible than the IDENTITY property. They are not bound to one table and you can use the same SEQUENCE object in multiple places if need be. They also give better performance.
Create a SEQUENCE object, specifying the data type (int), the starting value, and how much to increment by.
CREATE SEQUENCE dbo.IndexPositionSequence
AS int
START WITH 1
INCREMENT BY 1;
Then create the table, and use the NEXT VALUE FOR function to get a value from the SEQUENCE object.
CREATE TABLE dbo.SpeechOutputList
(
ID int NOT NULL IDENTITY(1,1),
IndexPosition int DEFAULT (NEXT VALUE FOR IndexPositionSequence),
SpeechConfigCode nvarchar(36) NOT NULL,
OutputSentence nvarchar(500),
PRIMARY KEY(ID)
)
Then we can insert some values into the table. Some of the values have IndexPosition specified and others not.
INSERT INTO dbo.SpeechOutputList (IndexPosition, SpeechConfigCode, OutputSentence)
VALUES
(123, N'abcd', N'The quick brown fox'),
(DEFAULT, N'efgh', N'jumped over the'),
(124, N'ijkl', N'lazy dog'),
(DEFAULT, N'mnop', N'and some cats');
Then display what's in the table.
SELECT *
FROM dbo.SpeechOutputList;
See MSDN > CREATE SEQUENCE (Transact-SQL): https://msdn.microsoft.com/en-us/library/ff878091.aspx

Answer 2

This is my suggestion
CREATE TABLE dbo.DataTable
(
ID int NOT NULL IDENTITY(1,1),
IndexPosition int,
Name varchar(10)
)
go
create trigger dbo.AI_DataTable on dbo.DataTable
after insert
as
begin
declare #id int = (select ID from inserted)
declare #ip int = (select IndexPosition from inserted)
print #id
print #ip
if (#ip is null)
begin
update dbo.DataTable
set IndexPosition = #id + 1
where ID = #id
end
end
-- you can implement any logic in trigger
-- Note: support bulk insert in trigger
insert into dbo.DataTable(Name) values ('First')
insert into dbo.DataTable (Name) values ('Second')
select * from dbo.DataTable
Result:
+----+---------------+--------+
| ID | IndexPosition | Name |
+----+---------------+--------+
| 1 | 2 | First |
| 2 | 3 | Second |
+----+---------------+--------+

Answer 3

I am not at the computer I did this on but I think I have it figured out
I tested this
You can have a Function as a default vaulue
Just have a function that returns the value
Make the function the default value
select isnull(max(IndexPosition),0) + 1 from table;