Question

If I have a table A with column INCREMENT_NUMBER. We will say that there are five rows.

1
2
3
4
4

If a user adds a new row to table A, the INCREMENT_NUMBER column of that row should be 5. (It could be anything, as long as it isn’t a number 1-4.) Keep in mind

INCREMENT_NUMBER integer auto_increment primary key

will not work because I need to allow for duplicates in the table and I don’t think

insert

Answer 1

If you want to allow duplicates, don’t declare it as a PRIMARY KEY (and neither put a UNIQUE constraint on it).

In MySQL an AUTO_INCREMENT is allowed to have duplicates in that case – you just have to add a simple index on it:

CREATE TABLE test
( increment_number INTEGER NOT NULL AUTO_INCREMENT
, INDEX inc_index (increment_number)
) ;

INSERT INTO test
VALUES (1),(2),(3),(4),(4);

Test in SQL-Fiddle: test-1

INSERT INTO test
VALUES
(NULL);

SELECT *
FROM test ;

Results in:

increment_number
----------------
1
2
3
4
4
5

---

In SQL-Server, you’ll have to toggle the IDENTITY_INSERT setting ON and OFF to have similar behaviour:

CREATE TABLE test
( increment_number INT IDENTITY(1,1) NOT NULL 
, name varchar(20) NOT NULL
) ;

SET IDENTITY_INSERT test ON ;

INSERT INTO test (increment_number, name)
VALUES 
(1, 'Alex'),
(2, 'Bill'),
(3, 'Cathy'),
(4, 'Diana'),
(4, 'Dean');

And then test-2:

SET IDENTITY_INSERT test OFF ;

INSERT INTO test (name)
VALUES
('Elaine') ;

SELECT *
FROM test ;

Results in:

increment_number  |  name
---------------------------
1                    Alex
2                    Bill
3                    Cathy
4                    Diana
4                    Dean
5                    Elaine