Looping on a relational database with Python

I'm trying to figure out a python loop that would find the minimum cost to go from point A to point B within a given dataset. For example, if someone wanted to fly from San Francisco to New York via the lowest cost with the below data.

sql.keys
# [u'id', u'departure', u'arrival', u'cost']

for i in sql:
  print i
#(0, u'San Francisco', u'New York', 600)
#(1, u'Denver', u'Chicago', 100)
#(2, u'New York', u'DC', 100)
#(3, u'Chicago', u'New York', 200)
#(4, u'San Francisco', u'Denver', 200)

With the above example, I'm hoping that the result will return the below:

(u'San Francisco', u'New York', 500)

Since someone can go between the two cities via SF -> Denver -> Chicago -> New York for 500 instead of flying direct for 600.

Thanks in advance!