# How to find the shortest distance/ travel time between two airports?

The problem was created by me, discussed with my co-workers and no one seem to have an idea. Thus, thought, I can ask the experts here.

I've the following table FlightInfo and the fields are

` Start Destination Flight_Duration `

The goal is to find out, the shortest flight between two cities. The challenge is not all cities have direct flights. (Example: PHL to PVA ->Philadelphia to Shangai). You have to connect in Detroit (DTW) or Chicago (ORD)

How will you go about writing an SQL statement?

Example table contents

PHL DTW 2.25

DTW PVG 15.15

PHL ORD 3.15

ORD PVG 16.20

-------------Problems Reply------------

Brute force:

```declare @t table ( [from] char(3), [to] char(3), [time] float);```

``` insert into @t ([from], [to], [time]) values ('PHL', 'DTW', 2.25), ('DTW', 'PVG', 15.15), ('PHL', 'ORD', 3.15), ('ORD', 'PVG', 16.20); declare @src char(3) = 'PHL', @dst char(3) = 'PVG'; ```

```with cteAnchor as ( select case @src when [from] then [to] when [to] then [from] end as [layover], [time] , [time] as [total] , cast([from]+'-'+[to] as varchar(max)) as [path] , 1 as [flights] from @t where @src in ([from], [to])) , cteRecursive as ( select [layover], [time], [total], [path], [flights] from cteAnchor union all select case r.layover when [from] then [to] when [to] then [from] end as [layover] , t.[time] , t.[time] + r.[total] as [total] , r.[path] + ' ' +t.[from]+'-'+t.[to] as [path] , r.[flights] + 1 from @t t join cteRecursive r on (t.[from] = r.[layover] and 0 = charindex(t.[to], r.[path])) or (t.[to] = r.[layover] and 0 = charindex(t.[from], r.[path])) ) select top(1) [flights], [total], [path] from cteRecursive where @dst = [layover] order by [total] asc; ```

Answer:

```total path 17.4 PHL-DTW DTW-PVG ```

Edit note: I have modified the implementation of the CTE with one which is resilient to cycles and is also actually correct.

I would make an assumption that you may get from any single airport to any another one in at most three flights. It is possible that it won't be true in few very exceptional cases, but is this really a problem? I don't think so, however you may consider adding another join if you feel you need it.

```Table flights: origin VARCHAR destination VARCHAR start DATETIME end DATETIME ```

And query:

```SELECT *, f3.end - f1.end AS duration FROM flights AS f1 INNER JOIN flights AS f2 ON f1.destination = f2.origin AND f1.end < f2.start INNER JOIN flights AS f3 ON f2.destination = f3.origin AND f2.end < f3.start WHERE f1.start BETWEEN some_reasonable_values_not_to_have_too_many_rows_in_join AND f2.start BETWEEN some_reasonable_values_not_to_have_too_many_rows_in_join AND f3.start BETWEEN some_reasonable_values_not_to_have_too_many_rows_in_join AND f1.origin = your_desired_origin AND f3.destination = your_desired_destination ORDER BY duration ASC LIMIT 1 ```

This is for combination of three flights. Similar SQL for two flights and one flight (less joins). Then, union those three queries and take the best result.

You may want to add some minimal delay between flights. `Some_reasonable_values_not_to_have_too_many_rows_in_join` - does it make sense to search flight combinations that take longer than e.g. three days?

Category:sql Views:3 Time:2010-08-09
Tags: sql sql server

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