How to sum in MySQL recursive query

Question

My problem statement is: I need to find places I can visit from Origin 'A' and their respective costs.

This is my table Train(Origin, Destination, LeastCost)

 +--------+-------------+------+
 | Origin | Destination | cost |
 +--------+-------------+------+
 | A      | B           |    1 |
 | A      | C           |    4 |
 | B      | C           |    2 |
 | A      | D           |    4 | 
 +--------+-------------+------+

I have tried a query:

with recursive Final(Origin, Destination, LeastCost) As(
-> (Select * from Train)
-> UNION
-> (Select T.Origin, F.Destination, F.LeastCost
-> from Train T, Final F
-> where T.Destination = F.Origin))
-> select * from Final ;

This gives me:

+--------+-------------+-----------+
| Origin | Destination | LeastCost |
+--------+-------------+-----------+
| A      | B           |         1 |
| A      | C           |         4 |
| B      | C           |         2 |
| A      | D           |         4 |
| A      | C           |         2 |
+--------+-------------+-----------+

The result I am looking for is

Origin | Destination | Price |
 A            C          3

As A-->B = 1, B-->C=2 , So A-->C=1+2=3 in the last row.

How do I achieve this? I tried using SUM(LeastCost) inside the recursive query but MySQl doesn't allow aggregations in there.

Answer 1

Add the two costs from the T and F aliases together in the recursive query. And then put additional logic in the final query to group the results:

with recursive Final(Origin, Destination, LeastCost) As(
    (Select * from Train)
    UNION
    (Select T.Origin, F.Destination, T.cost + F.LeastCost
     from Train T, Final F
     where T.Destination = F.Origin)
)
select   Origin, Destination, min(LeastCost)
from     Final
group by Origin, Destination

Through the recursive mechanism T.cost + F.LeastCost will make the cost sum up as you travel from one node through the tree to another.