Query with postgresql - ruby-on-rails

Question

Here below some sample 3 data, using PostgreSQL and rails
Stock
id: 42324
name: 'n1'
stock_items
id: 57889359
stock_id: 42324
check_id: 14123
turn: 5
mock_id: 57889357
id: 57889360
stock_id: 42324
check_id: 14141
turn: 3
mock_id: 0
Stock
id: 42325
name: 'n1'
stock_items
id: 57889361
stock_id: 42325
check_id: 19499
turn: 5
mock_id: 57889359
id: 57889362
stock_id: 42325
check_id: 19500
turn: 3
mock_id: 0
Here i have stock table and stock_items table i am trying to take the result like
if mock_id is 0 then get the check_id else other check_id whose mock_id is not zero.
So i tried one query
SELECT
check_id1,
CASE
WHEN stock_items.mock_id = 0 THEN stock_items.check_id
ELSE stock_items.check_id
END as check_id2
FROM
stock_items
INNER JOIN stocks on stocks.id = stock_items.stock_id ;
but the above query fails, but i need it like below, any suggestions?
check_id1 (mock_id=0) | check_id2 (mock_id !=0)
----------------------+-----------------------
14141 | 14123
19500 | 19499

Answer 1

CASE statement doesn't work like you think. You can read it here https://www.postgresql.org/docs/8.1/static/functions-conditional.html
Your query shoud look like this
SELECT s.check_id1, s2.check_id1 FROM stock_items s, (SELECT * FROM stock_items WHERE mock_id <> 0) s2 WHERE s.stock_id = s2.stock_id

Answer 2

What you're trying to do is basically a PIVOT. You should actually use two CASE, within an aggregation function, and having a GROUP BY (and an ORDER BY to get the order you gave):
SELECT
stock_id,
max(CASE when mock_id =0 then check_id END) AS "check_id1 (mock_id=0)",
max(CASE when mock_id<>0 then check_id END) AS "check_id2 (mock_id!=0)"
FROM
stock_items
INNER JOIN stocks on stocks.id = stock_items.stock_id
GROUP BY
stock_id
ORDER BY
stock_id;
stock_id | check_id1 (mock_id=0) | check_id2 (mock_id!=0)
-------: | --------------------: | ---------------------:
42324 | 14141 | 14123
42325 | 19500 | 19499
NOTE: I've added the stock_id so that the result is easier to understand. You can omit it if you don't actually want it in your response.
You can check all the setup and test it at dbfiddle here
UPDATE
As per comments
SELECT
stock_id,
-- This will get rid of the null in array_agg
ARRAY(SELECT
e
FROM
unnest(all_check_ids_with_mock_id_0) AS e
WHERE
e IS NOT NULL
) AS all_check_ids_with_mock_id_0,
ARRAY(SELECT
e
FROM
unnest(all_check_ids_with_mock_id_non_0) AS e
WHERE
e IS NOT NULL
) AS all_check_ids_with_mock_id_non_0
FROM
(
SELECT
stock_id,
array_agg(CASE WHEN mock_id =0 THEN check_id END) AS all_check_ids_with_mock_id_0,
array_agg(CASE WHEN mock_id<>0 THEN check_id END) AS all_check_ids_with_mock_id_non_0
FROM
stock_items
INNER JOIN stocks on stocks.id = stock_items.stock_id
GROUP BY
stock_id
) AS q0
ORDER BY
stock_id;
stock_id | all_check_ids_with_mock_id_0 | all_check_ids_with_mock_id_non_0
-------: | :--------------------------- | :-------------------------------
20223028 | {48752} | {194907,19260}
20223029 | {48743} | {194945,194907}
20223030 | {48752} | {194907}
20223031 | {48752} | {194907}
dbfiddle here