How to fetch only continuous records from mysql? - mysql

Question

I have a database table like below
___________
id | speed
-----------
1 | 3
2 | 2
3 | 0
4 | 0
5 | 0
6 | 2
7 | 0
8 | 0
9 | 2
10 | 0
Now I want to get the records where speed is 0 but only from 3 to 5 which are continuous and greater than any other continuous records. I don't want 7,8 records or the 10th record. How can I achieve this?

Answer 1

Probably the fastest method is to use MySQL session variables to increment the "group" each time the speed changes, as you scan through the rows.
select n.*, #groupid:=IF(#prev_speed=speed,#groupid,#groupid+1) as groupid, #prev_speed:=speed
from (select #groupid:=0, #prev_speed=-1) _init
cross join n
order by id;
+----+-------+---------+--------------------+
| id | speed | groupid | #prev_speed:=speed |
+----+-------+---------+--------------------+
| 1 | 3 | 1 | 3 |
| 2 | 2 | 2 | 2 |
| 3 | 0 | 3 | 0 |
| 4 | 0 | 3 | 0 |
| 5 | 0 | 3 | 0 |
| 6 | 2 | 4 | 2 |
| 7 | 0 | 5 | 0 |
| 8 | 0 | 5 | 0 |
| 9 | 2 | 6 | 2 |
| 10 | 0 | 7 | 0 |
+----+-------+---------+--------------------+
Then using the above query as a derived table, calculate the lowest and highest id per group, and the count of rows. Sort the groups by the count of rows.
select min(id) as minid, max(id) as maxid, count(*) as count
from (
select n.*, #groupid:=IF(#prev_speed=speed,#groupid,#groupid+1) as groupid, #prev_speed:=speed
from (select #groupid:=0, #prev_speed=-1) _init
cross join n
order by id
) as t1
group by t1.groupid
order by count desc;
+-------+-------+-------+
| minid | maxid | count |
+-------+-------+-------+
| 3 | 5 | 3 |
| 7 | 8 | 2 |
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 6 | 6 | 1 |
| 9 | 9 | 1 |
| 10 | 10 | 1 |
+-------+-------+-------+
Then using the first row from the above as another derived table, join to the original table for the rows in the range from the min to max id.
select n.*
from (
select min(id) as minid, max(id) as maxid, count(*) as count
from (
select n.*, #groupid:=IF(#prev_speed=speed,#groupid,#groupid+1) as groupid, #prev_speed:=speed
from (select #groupid:=0, #prev_speed=-1) _init
cross join n
order by id
) as t1
group by t1.groupid
order by count desc limit 1
) as t2
inner join n on n.id between t2.minid and t2.maxid
+----+-------+
| id | speed |
+----+-------+
| 3 | 0 |
| 4 | 0 |
| 5 | 0 |
+----+-------+