Doing a market basket analysis on the order details - sql

Question

I have a table that looks (abbreviated) like:
| order_id | item_id | amount | qty | date |
|---------- |--------- |-------- |----- |------------ |
| 1 | 1 | 10 | 1 | 10-10-2014 |
| 1 | 2 | 20 | 2 | 10-10-2014 |
| 2 | 1 | 10 | 1 | 10-12-2014 |
| 2 | 2 | 20 | 1 | 10-12-2014 |
| 2 | 3 | 45 | 1 | 10-12-2014 |
| 3 | 1 | 10 | 1 | 9-9-2014 |
| 3 | 3 | 45 | 1 | 9-9-2014 |
| 4 | 2 | 20 | 1 | 11-11-2014 |
I would like to run a query that would calculate the list of items
that most frequently occur together.
In this case the result would be:
|items|frequency|
|-----|---------|
|1,2, |2 |
|1,3 |1 |
|2,3 |1 |
|2 |1 |
Ideally, first presenting orders with more than one items, then presenting
the most frequently ordered single items.
Could anyone please provide an example for how to structure this SQL?

Answer 1

This query generate all of the requested output, in the cases where 2 items occur together. It doesn't include the last item of the requested output since a single value (2) technically doesn't occur together with anything... although you could easily add a UNION query to include values that happen alone.
This is written for PostgreSQL 9.3
create table orders(
order_id int,
item_id int,
amount int,
qty int,
date timestamp
);
INSERT INTO ORDERS VALUES(1,1,10,1,'10-10-2014');
INSERT INTO ORDERS VALUES(1,2,20,1,'10-10-2014');
INSERT INTO ORDERS VALUES(2,1,10,1,'10-12-2014');
INSERT INTO ORDERS VALUES(2,2,20,1,'10-12-2014');
INSERT INTO ORDERS VALUES(2,3,45,1,'10-12-2014');
INSERT INTO ORDERS VALUES(3,1,10,1,'9-9-2014');
INSERT INTO ORDERS VALUES(3,3,45,1,'9-9-2014');
INSERT INTO ORDERS VALUES(4,2,10,1,'11-11-2014');
with order_pairs as (
select (pg1.item_id, pg2.item_id) as items, pg1.date
from
(select distinct item_id, date
from orders) as pg1
join
(select distinct item_id, date
from orders) as pg2
ON
(
pg1.date = pg2.date AND
pg1.item_id != pg2.item_id AND
pg1.item_id < pg2.item_id
)
)
SELECT items, count(*) as frequency
FROM order_pairs
GROUP by items
ORDER by items;
output
items | frequency
-------+-----------
(1,2) | 2
(1,3) | 2
(2,3) | 1
(3 rows)

Answer 2

Market Basket Analysis with Join.
Join on order_id and compare if item_id < self.item_id. So for every item_id you get its associated items sold. And then group by items and count the number of rows for each combinations.
select items,count(*) as 'Freq' from
(select concat(x.item_id,',',y.item_id) as items from orders x
JOIN orders y ON x.order_id = y.order_id and
x.item_id != y.item_id and x.item_id < y.item_id) A
group by A.items order by A.items;