How do I add an integer that does not already exist? - sql

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