How to get the smallest Integers not yet in a database column - mysql

Question

I have a table in a MySQL DB with an UNIQUE INT(10) column. The table is pretty populated and the row contains non-consecutive entries of Integer numbers in that column. I would like to do a query, which gets me the smallest number (or the n smallest numbers) that is not in any row.
Example: The table contains rows with values (1, 2, 3, 5, 7, 8, 10, 12, 15) for the column. The sql statement should return i.e. the five lowest non-contained values, which are 4, 6, 9, 11, 13 in this case.
Is this possible with MySQL?

Answer 1

You can use a "numbers" table (it's handy for various operations):
CREATE TABLE num
( i UNSIGNED INT NOT NULL
, PRIMARY KEY (i)
) ;
INSERT INTO num (i)
VALUES
(1), (2), ..., (1000000) ;
Then:
SELECT
num.i
FROM
num
LEFT JOIN
tableX AS t
ON num.i = t.columnX
WHERE
t.columnX IS NULL
ORDER BY
num.i
LIMIT 5
or:
SELECT
num.i
FROM
num
WHERE
NOT EXISTS
( SELECT *
FROM tableX AS t
WHERE num.i = t.columnX
)
ORDER BY
num.i
LIMIT 5
Another approach, without using an auxilary table, would be to use MySQL variables. You can test it in SQL-Fiddle, test-2. The output is not the same as the previous (just to show that it can be done):
SELECT start_id, end_id
FROM
( SELECT
IF( t.columnX <> #id, #id, NULL) AS start_id
, IF( t.columnX <> #id, t.columnX-1, NULL) AS end_id
, #rows := #rows + (t.columnX - #id) AS r
, #id := t.columnX + 1 AS running_id
FROM
tableX AS t
CROSS JOIN
( SELECT #rows := 0
, #id := 1
) AS dummy
WHERE
#rows < 5
ORDER BY
t.columnX
) AS tmp
WHERE
start_id IS NOT NULL

Answer 2

This will work, but I think it is pretty inefficient. You won't need an extra table though (a table that would be (2^31-1)*4/1024^3 = 8GB for all positive numbers in INT). Also I advise you look at why you need this, because it might not be neccesary.
Also it will return the start and end of a range, but not all numbers in that range. (e.g. if you have numbers 1 and 5 it will return {0,2,4,6})
SELECT (t.num-1) AS bound FROM t
WHERE t.num-1 NOT IN (SELECT t.num FROM t)
UNION
SELECT (t.num+1) AS bound FROM t
WHERE t.num+1 NOT IN (SELECT t.num FROM t)
As I said this will be pretty inefficient, JOINs might be faster but you would need benchmark it.
SELECT (t.num-1) AS bound FROM t
LEFT JOIN t AS u ON t.num-1 = u.num
WHERE u.num IS NULL
UNION
SELECT (t.num+1) AS bound FROM t
LEFT JOIN t AS u ON t.num+1 = u.num
WHERE u.num IS NULL