create table natRules (
    id integer primary key asc,
    description varchar(255),
    externalPort integer not null,
    unique(externalPort)
)

##</code><code>##

1|value1|3221
2|value2|78
3|value3|32
4|value4|1114
5|value5|5
6|value6|1

##</code><code>##

SELECT  id
FROM    (
        SELECT  1 AS id
        ) q1
WHERE   NOT EXISTS
        (
        SELECT  1
        FROM    unused
        WHERE   id = 1
        )
UNION ALL
SELECT  *
FROM    (
        SELECT  id + 1
        FROM    unused t
        WHERE   NOT EXISTS
                (
                SELECT  1
                FROM    unused ti
                WHERE   ti.id = t.id + 1
                )
        ORDER BY
                id
        LIMIT 1
        ) q2
ORDER BY
        id
LIMIT 1