Getting count of rows with multiple combinations - mysql

Question

I need help with a MySQL query. We have a database (~10K rows) which I have simplified down to this problem.
We have 7 truck drivers who visit 3 out of a possible 9 locations, daily. Each day they visit exactly 3 different locations and each day they can visit different locations than the previous day. Here are representative tables:
Table: Drivers
id name
10 Abe
11 Bob
12 Cal
13 Deb
14 Eve
15 Fab
16 Guy
Table: Locations
id day address driver.id
1 1 Oak 10
2 1 Elm 10
3 1 4th 10
4 1 Oak 16
5 1 4th 16
6 1 Toy 16
7 1 Toy 11
8 1 5th 11
9 1 Law 11
10 2 Oak 11
11 2 4th 11
12 2 Toy 11
.........
We have data for a full year and we need to find out how many times each "route" is visited over a year, sorted from most to least.
From my high school math, I believe there are 9!/(6!3!) route combinations, or 84 in total. I want do something like:
Get count of routes where route addresses = 'Oak' and 'Elm' and '4th'
then run again
where route addresses = 'Oak' and 'Elm' and '5th'
then again and again, etc.Then sort the route counts, descending. But I don't want to do it 84 times. Is there a way to do this?

Answer 1

I'd be looking at GROUP_CONCAT
SELECT t.day
, t.driver
, GROUP_CONCAT(t.address ORDER BY t.address)
FROM mytable t
GROUP
BY t.day
, t.driver
What's not clear here, if there's an order to the stops on the route. Does the sequence make a difference, and how to we tell what the sequence is? To ask that a different way, consider these two routes:
('Oak','Elm','4th') and ('Elm','4th','Oak')
Are these equivalent (because it's the same set of stops) or are they different (because they are in a different sequence)?
If sequence of stops on the route distinguishes it from other routes with the same stops (in a different order), then replace the ORDER BY t.address with ORDER BY t.id or whatever expression gives the sequence of the stops.
Some caveats with GROUP_CONCAT: the maximum length is limited by the setting of group_concat_max_len and max_allowed_packet variables. Also, the comma used as the separator... if we combine strings that contain commas, then in our result, we can't reliably distinguish between 'a,b'+'c' and 'a'+'b,c'
We can use that query as an inline view, and get a count of the the number of rows with identical routes:
SELECT c.route
, COUNT(*) AS cnt
FROM ( SELECT t.day
, t.driver
, GROUP_CONCAT(t.address ORDER BY t.address) AS route
FROM mytable t
GROUP
BY t.day
, t.driver
) c
GROUP
BY c.route
ORDER
BY cnt DESC