Grouping the records on a specific criteria and to find the maximum value

Question

I have a veh_speed table with the fields vid, date_time, speed, status. My objective is to get the duration(start_date_time and end_date_time) of the vehicle with speed greater than 30. Currently I am generating the report using PL/SQL. Is it possilble to do with an SQL. Also it would be great if it is possible to get the max_speed between the range.

My table is as follows:

VID  START_DATE_TIME        SPEED  STATUS
---  -------------------    -----  ------
1   15/01/2014 10:00:05     0      N
1   15/01/2014 10:00:10    10      Y 
1   15/01/2014 10:00:15    30      Y
1   15/01/2014 10:00:20    35      Y
1   15/01/2014 10:00:25    45      Y
1   15/01/2014 10:00:27    10      Y
1   15/01/2014 10:00:29     0      Y
1   15/01/2014 10:00:30    20      Y
1   15/01/2014 10:00:35    32      Y
1   15/01/2014 10:00:40    33      Y
1   15/01/2014 10:00:45    35      Y
1   15/01/2014 10:00:50    38      Y
1   15/01/2014 10:00:55    10      Y

And I would like to get the following output:

VID   START_DATE_TIME          END_DATE_TIME          MAX_SPEED
---   ---------------          -------------          ---------
1    15/01/2014 10:00:15     15/01/2014 10:00:25      45
1    15/01/2014 10:00:35     15/01/2014 10:00:50      38

Here is the table creation script:

CREATE TABLE veh_speed(vid NUMBER(3), 
             date_time DATE, 
             speed NUMBER(3), 
             status CHAR(1));

INSERT ALL
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:05', 'dd/mm/yyyy hh24:mi:ss'),  0,  'N')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:10', 'dd/mm/yyyy hh24:mi:ss'), 10, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:15', 'dd/mm/yyyy hh24:mi:ss'), 30, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:20', 'dd/mm/yyyy hh24:mi:ss'), 35, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:25', 'dd/mm/yyyy hh24:mi:ss'), 45, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:27', 'dd/mm/yyyy hh24:mi:ss'), 10, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:29', 'dd/mm/yyyy hh24:mi:ss'),  0, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:30', 'dd/mm/yyyy hh24:mi:ss'), 20, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:35', 'dd/mm/yyyy hh24:mi:ss'), 32, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:40', 'dd/mm/yyyy hh24:mi:ss'), 33, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:45', 'dd/mm/yyyy hh24:mi:ss'), 35, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:50', 'dd/mm/yyyy hh24:mi:ss'), 38, 'Y')
     INTO veh_speed VALUES(1, to_date('15/01/2014 10:00:55', 'dd/mm/yyyy hh24:mi:ss'), 10, 'Y')
SELECT * FROM dual;

I hope I made my question clear.

Thanks in advance.

Answer 1

You can use analytic functions to group your records into blocks where the speed is 30 or more:

select vid, date_time, speed, status,
  case when speed >= 30 then 30 else 0 end as speed_limit,
  row_number() over (partition by vid order by date_time)
    - row_number() over (
      partition by vid, case when speed >= 30 then 30 else 0 end
      order by date_time) as chain
from veh_speed;

      VID DATE_TIME                SPEED STATUS SPEED_LIMIT      CHAIN
---------- ------------------- ---------- ------ ----------- ----------
         1 15/01/2014 10:00:05          0 N                0          0 
         1 15/01/2014 10:00:10         10 Y                0          0 
         1 15/01/2014 10:00:15         30 Y               30          2 
         1 15/01/2014 10:00:20         35 Y               30          2 
         1 15/01/2014 10:00:25         45 Y               30          2 
         1 15/01/2014 10:00:27         10 Y                0          3 
         1 15/01/2014 10:00:29          0 Y                0          3 
         1 15/01/2014 10:00:30         20 Y                0          3 
         1 15/01/2014 10:00:35         32 Y               30          5 
         1 15/01/2014 10:00:40         33 Y               30          5 
         1 15/01/2014 10:00:45         35 Y               30          5 
         1 15/01/2014 10:00:50         38 Y               30          5 
         1 15/01/2014 10:00:55         10 Y                0          7 

I can't take credit for the trick using two row_number() calls to generate chains of records, unfortunately, I picked that up somewhere (possibly here). The actual value of chain doesn't matter, just that they are unique within each vid and the same for all records in a contiguous block of records matching your criteria.

You're only interested in the chains of related records where the 'speed limit' was 30 (and that could just as easily be a Y/N flag or whatever), so you can use that and filter out those where the chain's speed was less than 30; and then use normal aggregate functios to get what you want:

select vid,
  min(date_time) as start_date_time,
  max(date_time) as end_date_time,
  max(speed) as max_speed
from (
  select vid, date_time, speed, status,
    case when speed >= 30 then 30 else 0 end as speed_limit,
    row_number() over (partition by vid order by date_time)
      - row_number() over (
        partition by vid, case when speed >= 30 then 30 else 0 end
        order by date_time) as chain
  from veh_speed
)
where speed_limit = 30
group by vid, chain
order by vid, start_date_time;

       VID START_DATE_TIME     END_DATE_TIME        MAX_SPEED
---------- ------------------- ------------------- ----------
         1 15/01/2014 10:00:15 15/01/2014 10:00:25         45 
         1 15/01/2014 10:00:35 15/01/2014 10:00:50         38 

SQL Fiddle.