Question

I am stumbled upon a problem, when I need a query which will produce a list of speeding time frames.

Here is the data example

[idgps_unit_location]   [dt]    [idgps_unit]    [lat]   [long]  [speed_kmh]
26  10/18/2012 18:53    2   47  56  30
27  10/18/2012 18:53    2   49  58  31
28  10/18/2012 18:53    2   28  37  15
29  10/18/2012 18:54    2   56  65  33
30  10/18/2012 18:54    2   152 161 73
31  10/18/2012 18:55    2   134 143 64
32  10/18/2012 18:56    2   22  31  12
36  10/18/2012 18:59    2   98  107 47
37  10/18/2012 18:59    2   122 131 58
38  10/18/2012 18:59    2   91  100 44
39  10/18/2012 19:00    2   190 199 98
40  10/18/2012 19:01    2   194 203 101
41  10/18/2012 19:02    2   182 191 91
42  10/18/2012 19:03    2   162 171 78
43  10/18/2012 19:03    2   174 183 83
44  10/18/2012 19:04    2   170 179 81
45  10/18/2012 19:05    2   189 198 97
46  10/18/2012 19:06    2   20  29  10
47  10/18/2012 19:07    2   158 167 76
48  10/18/2012 19:08    2   135 144 64
49  10/18/2012 19:08    2   166 175 79
50  10/18/2012 19:09    2   9   18  5
51  10/18/2012 19:09    2   101 110 48
52  10/18/2012 19:09    2   10  19  7
53  10/18/2012 19:10    2   32  41  20
54  10/18/2012 19:10    1   54  63  85
55  10/19/2012 19:11    2   55  64  50

I need a query that would convert this table into the following report that shows frames of time when speed was >80:

[idgps_unit]    [dt_start]  [lat_start] [long_start]    [speed_start]   [dt_end]    [lat_end]   [long_end]  [speed_end] [speed_average]
2   10/18/2012 19:00    190 199 98  10/18/2012 19:02    182 191 91  96.66666667
2   10/18/2012 19:03    174 183 83  10/18/2012 19:05    189 198 97  87
1   10/18/2012 19:10    54  63  85  10/18/2012 19:10    54  63  85  85

Now, what have I tried? I tried putting this into separate tables, queries and do some joins… Nothing works and I am very frustrated… I am not even sure if this could be done via the query. Asking for the expert help!

Answer 1

You were right, it is fairly tricky, but I think I’ve managed it:

SELECT  s.idgps_unit,
        MIN(s.dt) AS DT_Start,
        MIN(CASE WHEN s.RowNumber = 1 THEN s.Lat END) AS Lat_Start,
        MIN(CASE WHEN s.RowNumber = 1 THEN s.Long END) AS Long_Start,
        MIN(CASE WHEN s.RowNumber = 1 THEN s.Speed_kmh END) AS Speed_Start,
        MAX(s.dt) AS dt_end,
        MIN(CASE WHEN s.RowNumber = MaxRowNumber THEN s.Lat END) AS Lat_End,
        MIN(CASE WHEN s.RowNumber = MaxRowNumber THEN s.Long END) AS Long_End,
        MIN(CASE WHEN s.RowNumber = MaxRowNumber THEN s.Speed_kmh END) AS Speed_End,

        AVG(Speed_kmh) AS Speed_Average
FROM    (   SELECT  T.*,
                    @i:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN @i + 1 ELSE @i END AS IntervalID,
                    @r:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN 1 ELSE @r + 1 END AS RowNumber,
                    @b:= CASE WHEN Speed_Kmh> 80 THEN 1 ELSE 0 END AS IntervalCheck
            FROM    T,
                    (SELECT @i:= 0) i,
                    (SELECT @r:= 0) r,
                    (SELECT @b:= 0) b
            ORDER BY dt, idgps_unit_location
        ) s
        INNER JOIN
        (   SELECT  IntervalID, MAX(RowNumber) AS MaxRowNumber
            FROM    (   SELECT  T.*,
                                @i:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN @i + 1 ELSE @i END AS IntervalID,
                                @r:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN 1 ELSE @r + 1 END AS RowNumber,
                                @b:= CASE WHEN Speed_Kmh> 80 THEN 1 ELSE 0 END AS IntervalCheck
                        FROM    T,
                                (SELECT @i:= 0) i,
                                (SELECT @r:= 0) r,
                                (SELECT @b:= 0) b
                        ORDER BY dt, idgps_unit_location
                    ) d
            WHERE   IntervalCheck = 1
            GROUP BY IntervalID
        ) MaxInt
            ON MaxInt.IntervalID = s.IntervalID
WHERE   s.IntervalCheck = 1
GROUP BY s.IntervalID, s.idgps_unit;

The key is in this part:

SELECT  T.*,
        @i:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN @i + 1 ELSE @i END AS IntervalID,
        @r:= CASE WHEN Speed_Kmh > 80 AND @b = 0 THEN 1 ELSE @r + 1 END AS RowNumber,
        @b:= CASE WHEN Speed_Kmh> 80 THEN 1 ELSE 0 END AS IntervalCheck
FROM    T,
        (SELECT @i:= 0) i,
        (SELECT @r:= 0) r,
        (SELECT @b:= 0) b
ORDER BY dt, idgps_unit_location

Each time a row is encountered where the speed is over it sets the variable @b to 1, if this variable was 0 before it assigns the row a new intervalID, if it does this it begins numbering the row at 1 again, so you end up with something like this:

[idgps_unit_location]   [dt]                [idgps_unit]    [lat]   [long]  [speed_kmh] [IntervalID]    RowNumber   IntervalCheck
37                      10/18/2012 18:59    2               122     131     58          1               1           0
38                      10/18/2012 18:59    2               91      100     44          1               2           0
39                      10/18/2012 19:00    2               190     199     98          2               1           1
40                      10/18/2012 19:01    2               194     203     101         2               2           1
41                      10/18/2012 19:02    2               182     191     91          2               3           1
42                      10/18/2012 19:03    2               162     171     78          2               4           0
43                      10/18/2012 19:03    2               174     183     83          3               1           1

You then need to elimate all rows where the speed is under 80 (WHERE IntervalCheck = 1), and finally you can use aggregate functions along with CASE to find the rows where RowNumber is 1 (the first row of speeding), or the highest rownumber for that interval (the last row of speeding). The join at the end simply repeats the process to find what the maximum rownumber is for each intervalID.

Example on SQL Fiddle