mysql group by, max, count implementaion - mysql

Question

tables
items table
------------------
| id | item |
------------------
| 1 | pearl |
| 2 | ruby |
| 3 | gold |
| 4 | diamond |
------------------
transaction table
--------------------------------------------------
| item_id | sell_price | created_at |
--------------------------------------------------
| 1 | 10 | 2020-08-21 01:50:24 |
| 1 | 20 | 2020-08-22 05:51:10 |
| 3 | 20 | 2020-08-23 06:52:05 |
| 3 | 30 | 2020-08-24 22:55:07 |
| 4 | 50 | 2020-08-25 20:58:18 |
| 2 | 10 | 2020-08-26 12:59:55 |
--------------------------------------------------
expected output:
--------------------------------------------------------------------
| item_id | name | quantity | price | created_at |
--------------------------------------------------------------------
| 1 | pearl | 2 | 20 | 2020-08-22 05:51:10 |
| 2 | ruby | 1 | 10 | 2020-08-26 12:59:55 |
| 3 | gold | 2 | 30 | 2020-08-24 22:55:07 |
| 4 | diamond | 1 | 50 | 2020-08-25 20:58:18 |
--------------------------------------------------------------------
the query
SELECT
items.id AS item_id,
items.item,
COUNT(transaction.item_id) AS quantity,
MAX(transaction.price) AS sell_price
FROM transaction
LEFT JOIN items
ON transaction.item_id = items.id
GROUP BY transaction.item_id;
My query above works perfectly fine without transaction.created_at on select clause, but when I put select transaction.created_at it throws an error:
SELECT list is not in GROUP BY clause and contains nonaggregated column 'db.transaction.created_at' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

Answer 1

From the desired output you shared, it looks like you're trying to query the maximum created_at:
SELECT
items.id AS item_id,
items.item,
COUNT(transaction.item_id) AS quantity,
MAX(transaction.price) AS sell_price,
MAX(created_at) AS created_at -- Here!
FROM transaction
LEFT JOIN items
ON transaction.item_id = items.id
GROUP BY transaction.item_id;

Answer 2

When using GROUP BY, just list all columns that are in SELECT and that not inside an aggregate function.
SELECT
items.id AS item_id,
items.item,
COUNT(transaction.item_id) AS quantity,
MAX(transaction.sell_price) AS sell_price,
MAX(transaction.created_at) AS created_at
FROM transaction
LEFT JOIN items
ON transaction.item_id = items.id
GROUP BY items.id, items.item;
See db-fiddle.