Question

We have a database with a table whose values were imported from another system. There is an auto-increment column, and there aren’t any duplicate values, but there are missing values. For example, running this query:

select count(id) from arrc_vouchers where id between 1 and 100

should return 100, but it returns 87 instead. Is there a query I can run that will return the values of the missing numbers? For example, the records may exist for id 1-70 and 83-100, but there aren’t any records with id’s of 71-82. I want to return 71, 72, 73, etc.

Is this possible?

Answer 1

A better answer

JustPlainMJS provided a much better answer in terms of performance.

The (not as fast as possible) answer

Here’s a version that works on a table of any size (not just on 100 rows):

SELECT (t1.id + 1) as gap_starts_at,
       (SELECT MIN(t3.id) -1 FROM arrc_vouchers t3 WHERE t3.id > t1.id) as gap_ends_at
FROM arrc_vouchers t1
WHERE NOT EXISTS (SELECT t2.id FROM arrc_vouchers t2 WHERE t2.id = t1.id + 1)
HAVING gap_ends_at IS NOT NULL

• gap_starts_at – first id in current gap
• gap_ends_at – last id in current gap