How to track previous row status count - mysql

Question

I want to calculate count of order status changes within different states.
My Orderstatus table:
| id |ordr_id| status |
|----|-------|------------|
| 1 | 1 | pending |
| 2 | 1 | processing |
| 3 | 1 | complete |
| 4 | 2 | pending |
| 5 | 2 | cancelled |
| 6 | 3 | processing |
| 7 | 3 | complete |
| 8 | 4 | pending |
| 9 | 4 | processing |
Output I want:
| state | count |
|----------------------|-------|
| pending->processing | 2 |
| processing->complete | 2 |
| pending->cancelled | 1 |
Currently I'm fetching the results by SELECT order_id,GROUP_CONCAT(status) as track FROM table group by order_id and then process the data in php to get the output. But is that possible in query itself ?

Answer 1

Use lag():
select prev_status, status, count(*)
from (select t.*,
lag(status) over (partition by order_id order by status) as prev_status
from t
) t
group by prev_status, status;
LAG() is available in MySQL starting with version 8.
Note that you can filter out the first status for each order by putting where prev_status is not null in the outer query.
Your version is not quite correct, because it does not enforce the ordering. It should be:
SELECT order_id,
GROUP_CONCAT(status ORDER BY id) as track
EDIT:
In earlier versions of MySQL, you can use a correlated subquery:
select prev_status, status, count(*)
from (select t.*,
(select t2.status
from t t2
where t2.order_id = t.order_id and t2.id < t.id
order by t2.id desc
limit 1
) as prev_status
from t
) t
group by prev_status, status;

Answer 2

If id column ensure the sequence of records, you can use self join to achieve your requirement as below-
SELECT A.Status +'>'+ B.Status, COUNT(*)
FROM OrderStatus A
INNER JOIN OrderStatus B
ON A.id = B.id -1
WHERE B.Status IS NOT NULL
GROUP BY A.Status +'>'+ B.Status

Answer 3

With a join of the 3 status change types to the grouping of the table that you already did:
select c.changetype, count(*) counter
from (
select 'pending->processing' changetype union all
select 'processing->complete' union all
select 'pending->cancelled'
) c inner join (
select
group_concat(status order by id separator '->') changestatus
from tablename
group by ordr_id
) t on concat('->', t.changestatus, '->') like concat('%->', changetype, '->%')
group by c.changetype
See the demo.
Results:
> changetype | counter
> :------------------- | ------:
> pending->cancelled | 1
> pending->processing | 2
> processing->complete | 2

Answer 4

...or just a simple join...
SELECT CONCAT(a.status,'->',b.status) action
, COUNT(*) total
FROM my_table a
JOIN my_table b
ON b.ordr_id = a.ordr_id
AND b.id = a.id + 1
GROUP
BY action;
+----------------------+-------+
| action | total |
+----------------------+-------+
| pending->cancelled | 1 |
| pending->processing | 2 |
| processing->complete | 2 |
+----------------------+-------+
Note that this relies on the fact that ids are contiguous.