Compute an average number of transactions per user in a readable manner - mysql

Question

I have always been struggling with these types of queries. So, I'd like that someone checks my approach to handle those.
I am asked to find how many transactions, on average, each user executes during a 12 hours timespan starting from the first transaction.
This is the data:
CREATE TABLE IF NOT EXISTS `transactions` (
`transactions_ts` timestamp ,
`user_id` int(6) unsigned NOT NULL,
`transaction_id` bigint NOT NULL,
`item` varchar(200), PRIMARY KEY(`transaction_id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `transactions` (`transactions_ts`, `user_id`, `transaction_id`,`item` ) VALUES
('2016-06-18 13:46:51.0', 13811335,1322361417, 'glove'),
('2016-06-18 17:29:25.0', 13811335,3729362318, 'hat'),
('2016-06-18 23::07:12.0', 13811335,1322363995,'vase' ),
('2016-06-19 07:14:56.0',13811335,7482365143, 'cup'),
('2016-06-19 21:59:40.0',13811335,1322369619,'mirror' ),
('2016-06-17 12:39:46.0',3378024101,9322351612, 'dress'),
('2016-06-17 20:22:17.0',3378024101,9322353031,'vase' ),
('2016-06-20 11:29:02.0',3378024101,6928364072,'tie'),
('2016-06-20 18:59:48.0',13811335,1322375547, 'mirror');
My approach is the following (with the steps and the query itself below):
1) For each distinct user_id, find their first and 12 hours' transaction timestamp. This is accomplished by the inner query aliased as t1
2) Then, by inner join to the second inner query (t2), basically, I augment each row of the transactions table with two variables "first_trans" and "right_trans" of the 1st step.
3) Now, by where-condition, I select only those transaction timestamps that fall in the interval specified by first_trans and right_trans timestamps
4) Filtered table from the step 3 is now aggregated as count distinct transaction ids per user
5) The result of the 4 steps above is a table where each user has a count of transactions falling into the interval of 12 hrs from the first timestamp. I wrap it in another select that sums users' transaction counts and divides by the number of users, giving an average count per user.
I am quite certain that the end result is correct overall, but I keep thinking I might go without the 4th select. Or, perhaps, the whole code is somewhat clumsy, while my aim was to make this query as readable as possible, and not necessarily computationally optimal.
select
sum(dist_ts)/count(*) as avg_ts_per_user
from (
select
count(distinct transaction_id) as dist_ts,
us_id
from
(select
user_id as us_id,
min(transactions_ts) as first_trans,
min(transactions_ts) + interval 12 hour as right_trans
from transactions
group by us_id )
as t1
inner join
(select * from transactions )
as t2
on t1.us_id=t2.user_id
where transactions_ts >= first_trans
and transactions_ts < right_trans
group by us_id
) as t3
Fiddle demo

Answer 1

I don't think there is a mistake per se. The code can be slightly simplified (and neatened up a bit as follows):
select sum(dist_ts)/count(*) as avg_ts_per_user
from (
select count(distinct transaction_id) as dist_ts, us_id
from (
select user_id as us_id, min(transactions_ts) as first_trans, min(transactions_ts) + interval 12 hour as right_trans
from transactions
group by us_id
) as t1
inner join transactions as t2
on t1.us_id=t2.user_id and transactions_ts >= first_trans and transactions_ts < right_trans
group by us_id
) as t3
The (select * from transactions ) as t2 was simplified above and I somewhat arbitrarilly moved a where clause condition to the on clause of the inner join.
My Fiddle Demo
Here is a second way that does not use inner joins:
select sum(cnt)/count(*) as avg_ts_per_user from (
select count(*) as cnt, t.user_id
from transactions t
where t.transactions_ts >= (select min(transactions_ts) from transactions where user_id = t.user_id)
and t.transactions_ts < (select min(transactions_ts) + interval 12 hour from transactions where user_id = t.user_id)
group by t.user_id
) sq
Another Fiddle
You should probably run EXPLAIN against the two queries to see which one runs better on your server. Also note that min(transaction_ts) is specified twice for each user. Is MySql able to avoid the redundant calculation? I don't know. One possibility would be to create a temporary table consisting of user_id and min_transaction_ts so that the value is computed once. This would only make sense if your table had lots of rows and maybe not even then.