Question

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

Answer 1

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.