Find gaps of a sequence in SQL without creating additional tables

Question

I have a table invoices with a field invoice_number. This is what happens when i execute select invoice_number from invoice:

invoice_number
--------------
1
2
3
5
6
10
11

I want a SQL that gives me the following result:

gap_start | gap_end
4         | 4
7         | 9

How can i write a SQL to perform such query? I am using PostgreSQL.

Answer 1

With modern SQL, this can easily be done using window functions:

select invoice_number + 1 as gap_start, 
       next_nr - 1 as gap_end
from (
  select invoice_number, 
         lead(invoice_number) over (order by invoice_number) as next_nr
  from invoices
) nr
where invoice_number + 1 <> next_nr;

SQLFiddle: http://sqlfiddle.com/#!15/1e807/1