Ask the Expert

Gaps in sequences

My table has one column of type integer. The data within that column is a simple sequence of numbers. If I have a number of rows with the values incrementing by one, what SQL could I use to determine if there is a skip in sequence?

For example, in my one column, I have 999 rows with the integer values of 1 to 1000, excluding 500. How can I return an SQL result that has one row with the value of 500?

    Requires Free Membership to View

The easiest way is to join the table to itself. Join each row to the one with the next id, by matching + 1 = Use an outer join, and look for unmatched rows, i.e. where is null:

select + 1 as missing
  from onethousand t1
left outer
  join onethousand t2
    on + 1 =
 where is null      


That's the result you wanted, except for the 1001, which is from the row with the highest id in the table. In other words, 500 is missing from within the sequence, and 1001 is the first number missing after the sequence. You could remove this with:

   and < (select max(id) from onethousand)

The + 1 = self-join method will actually only report the first number in a gap in the sequence. If you were to delete the 501 and 502 rows, the outer join query will still only report 500 as missing. When matching + 1 =, 500 matches 499, but 501, 502, and 503 never come up because 500, 501, and 502 are missing from the t1 side of the join. This is easily seen with this query:

select     as t1id
     , + 1 as t1idplus1
     ,     as t2id
  from onethousand t1
left outer
  join onethousand t2
    on +1 = 
 where between 497 and 504
order by    

t1id  t1idplus1  t2id  
497   498        498 
498   499        499 
499   500        NULL 
503   504        504 
504   505        505 

If you needed to itemize all the missing numbers, you could do that with a left outer join from The integers table.

This was first published in February 2004

There are Comments. Add yours.

TIP: Want to include a code block in your comment? Use <pre> or <code> tags around the desired text. Ex: <code>insert code</code>

REGISTER or login:

Forgot Password?
By submitting you agree to receive email from TechTarget and its partners. If you reside outside of the United States, you consent to having your personal data transferred to and processed in the United States. Privacy
Sort by: OldestNewest

Forgot Password?

No problem! Submit your e-mail address below. We'll send you an email containing your password.

Your password has been sent to: