====== Find gaps or duplicates in number sequences in MySql ======
<code sql Find all gaps>
select l.id + 1 as start
    from sequence as l 
    left outer join sequence as r on l.id + 1 = r.id
    where r.id is null
</code>

<code sql Find first gap>
select min(l.id + 1) as start 
    from sequence as l 
    left outer join sequence as r on l.id + 1 = r.id
    where r.id is null
</code>

<code sql Find missing range>
select start, stop from (
    select m.id + 1 as start, (select min(id) - 1 from sequence as x where x.id > m.id) as stop
        from sequence as m
        left outer join sequence as r on m.id = r.id - 1
        where r.id is null
    ) as x
    where stop is not null;
</code>

<code sql Find duplicates>
select id, count(*) from 
    sequence 
    group by id having count(*) > 1;
</code>