mysql avg length of a date squence - mysql

Question

I have a report i'm trying to figure out, but I would like to do it all with in a SQL statement instead of needing to iterate over a bunch of data in script to do it.
I have a table that is structured like:
CREATE TABLE `batch_item` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`record_id` int(11) DEFAULT NULL,
`created` DATE NOT NULL,
PRIMARY KEY (`id`),
KEY `record_id` (`record_id`)
);
The Date field is always YEAR-MONTH-01. Data looks something like:
+------+-----------+------------+
| id | record_id | created |
+------+-----------+------------+
| 1 | 1 | 2019-01-01 |
| 2 | 2 | 2019-01-01 |
| 3 | 3 | 2019-01-01 |
| 4 | 1 | 2019-02-01 |
| 5 | 2 | 2019-02-01 |
| 6 | 1 | 2019-03-01 |
| 7 | 3 | 2019-03-01 |
| 8 | 1 | 2019-04-01 |
| 9 | 2 | 2019-04-01 |
+------+-----------+------------+
So what I'm trying to do, with out having to create a looping script, is find the AVG number of sequential months for each record. Example with the data above would be:
Record_id 1 would have a avg of 4 months.
Record_id 2 would be 1.5
Record_id 3 would be 1
I can write a script to iterate through all the records. I just would rather avoid that.

Answer 1

This is a gaps-and-islands problem. You simply need an enumeration of the rows for this to work. In MySQL 8+, you would use row_number() but you can use a global enumeration here:
select record_id, min(created) as min_created, max(created) as max_created, count(*) as num_months
from (select bi.*, (created - interval n month) as grp
from (select bi.*, (#rn := #rn + 1) as n -- generate some numbers
from batch_item bi cross join
(select #rn := 0) params
order by bi.record_id, bi.month
) bi
) bi
group by record_id, grp;
Note that when using row_number(), you would normally partition by record_id. However that is not necessary, if the numbers are created in the correct sequence.
The above query gets the islands. For your final results, you need one more level of aggregation:
select record_id, avg(num_months)
from (select record_id, min(created) as min_created, max(created) as max_created, count(*) as num_months
from (select bi.*, (created - interval n month) as grp
from (select bi.*, (#rn := #rn + 1) as n -- generate some numbers
from batch_item bi cross join
(select #rn := 0) params
order by bi.record_id, bi.month
) bi
) bi
group by record_id, grp
) bi
group by record_id;

Answer 2

This is not a tested solution. It should work in MySQL 8.x with minor tweaks, since I don't remember date arithmetic in MySQL:
with
a as ( -- the last row of each island
select *
from batch_item
where lead(created) over(partition by record_id order by created) is null
or lead(created) over(partition by record_id order by created)
> created + 1 month -- Fix the date arithmetic here!
),
e as ( -- each row, now with the last row of its island
select b.id, b.record_id, min(a.last_created) as end_created
from batch_item b
join a on b.record_id = a.record_id and b.created <= a.created
group by b.id, b.record_id
),
m as ( -- each island with the number of months it has
select
record_id, end_created, count(*) as months
from e
group by record_id, end_created
)
select -- the average length of islands for each record_id
record_id, avg(months) as avg_months
from m
group by record_id