列出连续记录范围的有效方法

Question

I have a table set up like so:

CREATE TABLE `cn` (
    `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `type` int(3) unsigned NOT NULL,
    `number` int(10) NOT NULL,
    `desc` varchar(64) NOT NULL,
    `datetime` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (`id`)
) ENGINE=InnoDB

number is usually but not necessarily unique.

Most of the table consists of rows with consecutive number entries.

eg

101010, 101011, 101012, etc.

I've been trying to find an efficient way to list ranges of consecutive numbers so I can find out where numbers are "missing" easily. What I'd like to do is list the start number, end number, and number of consecutive rows. Since there can be duplicates, I am using SELECT DISTINCT(number) to avoid duplicates.

I've not been having much luck - most of the questions of this type deal with dates and have been hard to generalize. One query was executing forever, so that was a no go. This answer is sort of close but not quite. It uses a CROSS JOIN , which sounds like a recipe for disaster when you have millions of records.

What would the best way to do this be? Some answers use joins, which I'm skeptical of performance wise. Right now there are only 50,000 rows, but it will be millions of records within a few days, and so every ounce of performance matters.

The eventual pseudoquery I have in mind is something like:

SELECT DISTINCT(number) FROM cn WHERE type = 1 GROUP BY [consecutive...] ORDER BY number ASC

Answer 1

This is a gaps-and-islands problem. You can solve by using the difference between row_number() and number to define groups; gaps are identified by changes in the difference:

select type, min(number) first_number, max(number) last_number, count(*) no_records
from (
    select cn.*, row_number() over(order by number) rn
    from cn
    where type = 1
) c
group by type, number - rn

Note: window functions avalailable in MySQL 8.0 and MariaDB 10.3 onwards.

---

In earlier versions, you can emulate row_number() with a session variable:

select type, min(number) first_number, max(number) last_number, count(*) no_records
from (
    select c.*, @rn := @rn + 1 rn
    from (select * from cn where type = 1 order by number) c
    cross join (select @rn := 0) r
) c
group by number - rn