SQL query to find all combinations of grouped values - sql

Question

I am looking for a SQL query or a series of SQL queries.
Schema
I have a logging table with three columns: id, event_type, and timestamp
The IDs are arbitrary text, generated randomly at runtime and unknown to me
The event types are numbers from a finite collection of known event types
The timestamps are your typical int64 epoch timestamp
A single ID value may have 1 or more rows, each with some value for event_type. representing a flow of events associated with the same ID
For each ID, its collection of rows can be sorted by increasing timestamp
Most times, there will be only one occurrence of an ID + event type combination, but rarely, there could be two; not sure this matters
Goal
What I want to do is to query the number of distinct combinations of event types (sorted by timestamp). For example, provided this table:
id event_type timestamp
-----------------------------------------
foo event_1 101
foo event_2 102
bar event_2 102
bar event_1 101
foo event_3 103
bar event_3 103
blah event_1 101
bleh event_2 102
backwards event_1 103
backwards event_2 102
backwards event_3 101
Then I should get the following result:
combination count
-------------------------------
[event_1,event_2,event_3] 2 // foo and bar
[event_3,event_2,event_1] 1 // backwards
[event_1] 1 // blah
[event_2] 1 // bleh

Answer 1

You can do 2 levels of grouping to your data.
For Mysql use group_concat():
select t.combination, count(*) count
from (
select
group_concat(event_type order by timestamp) combination
from tablename
group by id
) t
group by t.combination
order by count desc
See the demo.
For Postgresql use array_agg() with array_to_string():
select t.combination, count(*) count
from (
select
array_to_string(array_agg(event_type order by timestamp), ',') combination
from tablename
group by id
) t
group by t.combination
order by count desc
See the demo.
For Oracle there is listagg():
select t.combination, count(*) count
from (
select
listagg(event_type, ',') within group (order by timestamp) combination
from tablename
group by id
) t
group by t.combination
order by count desc
See the demo.
For SQL Server 2017+ there is string_agg():
select t.combination, count(*) count
from (
select
string_agg(event_type, ',') within group (order by timestamp) combination
from tablename
group by id
) t
group by t.combination
order by count desc
See the demo.
Results:
| combination | count |
| ----------------------- | ----- |
| event_1,event_2,event_3 | 2 |
| event_3,event_2,event_1 | 1 |
| event_1 | 1 |
| event_2 | 1 |

Answer 2

SELECT
"combi"."combination",
COUNT(*) AS "count"
FROM
(
SELECT
GROUP_CONCAT("event_type" SEPARATOR ',') AS "combination"
FROM
?table?
GROUP BY
"id"
) AS "combi"
GROUP BY
"combi"."combination"
Note: GROUP_CONCAT(... SEPARATOR ...) syntax is not SQL standard, it's DB specific (in this case MySQL, other dbs have other aggregate functions). You might need to adjust for your DB of choice or specify in tags which DB you are actually using.
As for "sorted by timestamp", you need to define what this actually means. What is "sorted by timestamp" for a group of groups?