How to count on a group by array_agg in PostgreSQL

Question

Here is my SQL: SELECT t.uid, array_agg(t.place) FROM tour_tracking t WHERE (orderon::time BETWEEN '18:00:00' AND '20:00:00') GROUP BY t.uid;

Origin result:
|---------------|----------------------| | uid | place | |---------------|----------------------| | a01 | {hk, hk, jp} | |---------------|----------------------| | a02 | {jp, jp, jp, jp, uk} | |---------------|----------------------|

Now I want to count on each DISTINCT place for every group-by-ed uid. Hopeful result:

|---------------|--------------------------------------| | uid | place | |---------------|--------------------------------------| | a01 | something like this: {hk,2, jp,1} | |---------------|--------------------------------------| | a02 | {jp:4, uk:1} | |---------------|--------------------------------------|
I try to combine some count() sql query but won't work.., how to do the right query?

PostgreSQL version: 10.3

Answer 1

Aggregate twice. Once to get the places and their counts, then again to make the lists...

SELECT
  t.uid,
  array_agg(array[t.place, t.row_count])
FROM
(
  SELECT
    uid,
    place,
    COUNT(*)   AS row_count
  FROM
    tour_tracking
  WHERE
    orderon::time BETWEEN '18:00:00' AND '20:00:00'
  GROUP BY
    uid,
    place
)
  t
GROUP BY
  t.uid