How should I calculate the average speed by road segment for multiple segments?

Question

I have a table of driver speeds and road segments:

driver_lpr    |   segment    |    speed
  0000001     |       A      |     30
  0000002     |       B      |           


        

Answer 1

This is little bit complex, but will take care of 0 speed also. I have 2 similar queries to do it based on different scenarios.

Assume your source table like below.

+-------------+----------+-------+
| driver_lpr  | segment  | speed |
+-------------+----------+-------+
|    0000001  | A        |    30 |
|    0000002  | B        |    60 |
|    0000003  | A        |    50 |
|    0000004  | A        |   100 |
|    0000005  | B        |    60 |
|    0000006  | B        |     0 |
|    0000007  | C        |     0 |
+-------------+----------+-------+

I have added 2 new rows with 0 speed.

Case 1:

• Addition of a 0 speed in segment B, will give average speed as 40 (60*2+0*1)/(2+1).
• Addition of a new segment, C with 0 speed will give average speed as 0

So output would be

+----------+-------------------+
| segment  |   average_speed   |
+----------+-------------------+
| A        | 47.36842105263158 |
| B        | 40                |
| C        | 0                 |
+----------+-------------------+

SQLFiddle Demo CASE 1

Case 2:

• There will be no change in average speed of B with addition of a 0.
• However, a new segment C will have 0 average speed.

Output would be

+----------+-------------------+
| segment  |   average_speed   |
+----------+-------------------+
| A        | 47.36842105263158 |
| B        | 60                |
| C        | 0                 |
+----------+-------------------+

SQLFiddle Demo CASE 2

Query for Case 1:

with tbl1 as
    (SELECT segment,
    case when speed = 0 then cast(0 as float) else 
    cast(1 as float)/cast(speed as float)
    end as newspeed
    FROM T 
    ),
tbl2 as
    (
        select segment,cast(1 as float)/avg(newspeed) as avgspeed,count(*) as cnt 
        from tbl1
        where newspeed <> 0
        group by segment
    union
        select segment,0 as avgspeed,count(*) as cnt
        from tbl1
        where newspeed =0
        group by segment
    )
select segment,
    sum(avgspeed*cnt)/sum(cnt) as "average_speed" 
from tbl2
group by segment

Query for Case2

with tbl1 as
    (SELECT segment,
    case when speed = 0 then cast(0 as float) else 
    cast(1 as float)/cast(speed as float)
    end as newspeed
    FROM T 
    ),
tbl2 as
    (
        select segment,cast(1 as float)/avg(newspeed) as avgspeed,count(*) as cnt 
        from tbl1
        where newspeed <> 0
        group by segment
    union
        select segment,0 as avgspeed,count(*) as cnt
        from tbl1
        where newspeed =0
        group by segment
    )
select segment,
    sum(avgspeed) as "average_speed" 
from tbl2
group by segment