self join after an inner join - sql

Question

I am finding what cities have the same name in different states. The city name and state name are in seperate tables (cities and states) and can be inner joined over a seperate common column.
select c1.city, c1.state, c2.city, c2.state
from cities
inner join states on cities.commonid = states.commonid
After inner joining i need to self join to perform a function as follows
select c1.city, c1.state, c2.city, c2.state
from *joined table* c1 join
*joined table* c2
on c1.city = c2.city and c1.state <> c2.state
i am wondering how i can self join a table that is the result of another join in the same query
output will be like this
+----------+-------+--------+--------+
| city1 | state1|city2 |state2 |
+----------+-------+--------+--------+
| x | melb | x | syd |
| y | bris | y | ACT |
+----------+-------+--------+--------+

Answer 1

I assume that the table cities has a column like state_id that references a column state_id in the table states (change the names to the actual names of the columns).
First do a self join for cities with the conditions:
c1.city = c2.city AND c1.state_id < c2.state_id
The < operator makes sure that each pair of cities wil be returned only once.
Then join 2 copies of states, because each of them will be used to get the name of the state for each of the 2 cities:
SELECT c1.city city1, s1.state state1,
c2.city city2, s2.state state2
FROM cities c1
INNER JOIN cities c2 ON c1.city = c2.city AND c1.state_id < c2.state_id
INNER JOIN states s1 ON s1.state_id = c1.state_id
INNER JOIN states s2 ON s2.state_id = c2.state_id
ORDER BY city1

Answer 2

You can do select of your inner join query and give it alias.
Then it will become something like below...
Select c.city,c.state from
(Select City,state from cities inner join states where cities.id = states.id) as c
Now make a self join for c.

Answer 3

I would perform a pre-query of all cities that have more than one state. Then join to the states table to see which states they are encountered.
select
Duplicates.city,
s.state
from
( select c1.city
from cities c1
group by c1.city
having count(*) > 1 ) Duplicates
JOIN cities c2
on Duplicates.city = c2.city
JOIN states s
on c2.commonid = s.commonid
order by
Duplicates.city,
s.state
By NOT trying to do cross-tab of just two city/states, you would get a single list. If one city name exists in 5 states, how would you plan on showing that. This way you would see all alphabetized.

Answer 4

I would suggest a CTE:
with cs as (
select c.name as city_name, s.name as state_name
from cities c join
states s
on c.commonid = s.commonid
)
select cs1.*, cs2.*
from cs cs1 join
cs cs2
on cs1.name = cs2.name and cs1.state <> cs2.state;