Possible to implement a manual increment with just

Question

I have a primary key that I don't want to auto increment (for various reasons) and so I'm looking for a way to simply increment that field when I INSERT. By simply, I mean without stored procedures and without triggers, so just a series of SQL commands (preferably one command).

Here is what I have tried thus far:

BEGIN TRAN

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

COMMIT TRAN;

* Data abstracted to use generic names and identifiers

However, when executed, the command errors, saying that

"Subqueries are not allowed in this context. only scalar expressions are allowed"

So, how can I do this/what am I doing wrong?

---

EDIT: Since it was pointed out as a consideration, the table to be inserted into is guaranteed to have at least 1 row already.

Answer 1

How about creating a separate table to maintain the counter? It has better performance than MAX(id), as it will be O(1). MAX(id) is at best O(lgn) depending on the implementation.

And then when you need to insert, simply lock the counter table for reading the counter and increment the counter. Then you can release the lock and insert to your table with the incremented counter value.

Answer 2

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.

Answer 3

declare @nextId int
set @nextId = (select MAX(id)+1 from Table1)

insert into Table1(id, data_field) values (@nextId, '[blob of data]')

commit;

But perhaps a better approach would be using a scalar function getNextId('table1')

Answer 4

I don't know if somebody is still looking for an answer but here is a solution that seems to work:

-- Preparation: execute only once
    CREATE TABLE Test (Value int)

CREATE TABLE Lock (LockID uniqueidentifier)
INSERT INTO Lock SELECT NEWID()

-- Real insert

    BEGIN TRAN LockTran

    -- Lock an object to block simultaneous calls.
    UPDATE  Lock WITH(TABLOCK)
    SET     LockID = LockID

    INSERT INTO Test
    SELECT ISNULL(MAX(T.Value), 0) + 1
    FROM Test T

    COMMIT TRAN LockTran

Answer 5

This should work:

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

Or this (substitute LIMIT for other platforms):

INSERT INTO Table1 (id, data_field)
SELECT TOP 1
    MAX(id) + 1, '[blob of data]'
FROM
   Table1
ORDER BY
   [id] DESC;

Answer 6

We have a similar situation where we needed to increment and could not have gaps in the numbers. (If you use an identity value and a transaction is rolled back, that number will not be inserted and you will have gaps because the identity value does not roll back.)

We created a separate table for last number used and seeded it with 0.

Our insert takes a few steps.

--increment the number Update dbo.NumberTable set number = number + 1

--find out what the incremented number is select @number = number from dbo.NumberTable

--use the number insert into dbo.MyTable using the @number

commit or rollback

This causes simultaneous transactions to process in a single line as each concurrent transaction will wait because the NumberTable is locked. As soon as the waiting transaction gets the lock, it increments the current value and locks it from others. That current value is the last number used and if a transaction is rolled back, the NumberTable update is also rolled back so there are no gaps.

Hope that helps.

Another way to cause single file execution is to use a SQL application lock. We have used that approach for longer running processes like synchronizing data between systems so only one synchronizing process can run at a time.