IN list in subquery and mainquery leads to duplication - postgresql

Question

I started out with a query that gives me all shifts for a person.
SELECT
p.id as person_id, json_agg(sh) as shifts
FROM
people as p,
(SELECT s.id, s.date_of_shift, s.shift_type_id
FROM people as p
LEFT JOIN shifts as s
ON s.person_id = p.id AND s.date_of_shift BETWEEN '2016-02-11' AND '2016-02-17'
WHERE p.id = 2001
ORDER BY p.id
) as sh
WHERE
p.id = 2001
GROUP BY
p.id
;
the result would be something like this:
person_id | shifts
-----------+------------------------------------------------------------------
2001 | [{"id":580069,"date_of_shift":"2016-02-11","shift_type_id":44},+
{"id":580070,"date_of_shift":"2016-02-12","shift_type_id":42}, +
{"id":580071,"date_of_shift":"2016-02-15","shift_type_id":49}, +
{"id":580072,"date_of_shift":"2016-02-16","shift_type_id":41}, +
{"id":580073,"date_of_shift":"2016-02-17","shift_type_id":48}]
so I got 1 row, the first column is the person id and the 2nd is the json with the array of shifts.
The next step would be, to give the query a list of person_ids and get something like this
person_id | shifts
----------|--------------
2001 | [{..},]
2002 | [{..},]
2003 | [{..},]
so I ran this:
SELECT
p.id as person_id, json_agg(sh) as shifts
FROM
people as p,
(SELECT s.id, s.date_of_shift, s.shift_type_id
FROM people as p
LEFT JOIN shifts as s
ON s.person_id = p.id AND s.date_of_shift BETWEEN '2016-02-11' AND '2016-02-17'
WHERE p.id IN (2201,2202,2203)
ORDER BY p.id
) as sh
WHERE
p.id IN (2201,2202,2203)
GROUP BY
p.id
;
The problem is that I now get all shifts for every person inside the subquery.
So in this case I get 15 shifts for every person_id, instead of 5.
I understand why I get this result, but I'm stuck on how to get the result I'm look for.

Answer 1

You can use a LATERAL join:
SELECT p.id as person_id, json_agg(sh) as shifts
FROM people as p,
LATERAL (
SELECT s.person_id, s.date_of_shift, s.shift_type_id
FROM shifts as s
WHERE s.person_id = p.id AND
s.date_of_shift BETWEEN '2016-02-11' AND '2016-02-17') as sh
WHERE p.id IN (2201,2202,2203)
GROUP BY p.id;
This way the subquery is simplified: you don't to perform a LEFT JOIN operation inside it, as you can access table people that lies outside the scope of the subquery.
Demo here
If you want to always get all rows of table people irrespective if there are matching rows in shifts table, then you can use a LEFT JOIN LATERAL:
SELECT p.id as person_id, json_agg(sh) as shifts
FROM people as p
LEFT JOIN LATERAL (
SELECT person_id, date_of_shift, shift_type_id
FROM shifts) as sh
ON sh.person_id = p.id AND
sh.date_of_shift BETWEEN '2016-02-11' AND '2016-02-17'
WHERE p.id IN (2201,2202,2203, 2204)
GROUP BY p.id;
Demo here