sql to select first n unique lines on sorted result - sql

Question

I have query resulting me 1 column of strings, result example:
NAME:
-----
SOF
OTP
OTP
OTP
SOF
VIL
OTP
SOF
GGG
I want to be able to get SOF, OTP, VIL - the first 3 unique top,
I tried using DISTINCT and GROUP BY, but it is not working, the sorting is damaged..
The query building this result is :
SELECT DISTINCT d.adst
FROM (SELECT a.date adate,
b.date bdate,
a.price + b.price total,
( b.date - a.date ) days,
a.dst adst
FROM flights a
JOIN flights b
ON a.dst = b.dst
ORDER BY total) d
I have "flights" table with details, and I need to get the 3 (=n) cheapest destinations.
Thanks

Answer 1

This can easily be done using window functions:
select *
from (
SELECT a.date as adate,
b.date as bdate,
a.price + b.price as total,
dense_rank() over (order by a.price + b.price) as rnk,
b.date - a.date as days,
a.dst as adst
FROM flights a
JOIN flights b ON a.dst = b.dst
) t
where rnk <= 3
order by rnk;
More details on window functions can be found in the manual:
http://www.postgresql.org/docs/current/static/tutorial-window.html

Answer 2

Find a way to do it.
I am selecting the DST and the PRICE, grouping by DST with MIN function on Price and limiting 3.
do I have better way to do it?
SELECT d.adst , min(d.total) mttl
FROM (SELECT a.date adate,
b.date bdate,
a.price + b.price total,
( b.date - a.date ) days,
a.dst adst
FROM flights a
JOIN flights b
ON a.dst = b.dst
ORDER BY total) d
group by adst order by mttl;

Answer 3

select
name
from
testname
where
name in (
select distinct(name) from testname)
group by name order by min(ctid) limit 3
SQLFIDDLE DEMO

Answer 4

You can tweak your query to return the correct result, by adding where days > 0 and limit 3 in the outer query like this:
select *
from
(
select
a.date adate,
b.date bdate,
(a.price + b.price) total,
(b.date - a.date) days ,
a.dst adst
from flights a
join flights b on a.dst = b.dst
order by total
) d
where days > 0
limit 3;
SQL Fiddle Demo
This assuming that the second entry is the return flight with date greater than the first entry. So that you got positive days difference.
Note that, your query without days > 0 will give you a cross join between the table and it self, for each flight you will get 4 rows, two with it self with days = 0 and other row with negative days so I used days > 0 to get the correct row.
I recommend that you add a new column, an Id Flight_Id as a primary key, and another foreign key something like From_Flight_Id. So the primary flight would have a null From_Flight_Id, and the returning flight will have a From_Flight_Id equal to the flight_id of the primary filght, this way you can join them properly instead.

Answer 5

SELECT DISTINCT(`EnteredOn`) FROM `rm_pr_patients` Group By `EnteredOn`

Answer 6

SELECT DISTINCT ON (column_name) FROM table_name order by name LIMIT 3;