SQL - JOIN with MAX(created_at) - mysql

Question

Problem
I have 3 tables: customers, companies and calls.
A customer has many Companies, and a Company has many calls. (both one to many).
The current status of a Company is the result of the last call for the company (MAX(created_at)).
Now I want a list of all companies for a customer with the columns of the last call in the results.
Needed Result
The result should be:
company.*, lastcall.*,
There could be calls with the same created_at date. Then there should be only 1 row in de result.
Not all companies has a call yet, the company should still be in the result and the columns of the call should be NULL. (left join)
Tables
customers
- id (int, primary key)
- name (varchar)
- address (varchar)
- city (varchar)
companies
- id (int, primary key)
- customer_id (int)
- name (varchar)
- address (varchar)
- city (varchar)
calls
- id (int, primary key)
- company_id (int)
- result (varchar)
- created_at (datetime)
Attempt
A query which didn't work I came up with is:
SELECT * FROM companies co
LEFT JOIN calls ca ON co.id = ca.company_id
WHERE co.customer_id = ?
GROUP BY co.id
HAVING ca.created_at = (SELECT max(ll.created_at) FROM calls ll WHERE ll.company_id = co.id)

Answer 1

you should just join a select so that way you arent trying to re evaluate the select.
SELECT co.id, co.label, ca.result, ca.id, t.date_created as most_recent
FROM companies co
LEFT JOIN
( SELECT MAX(created_at) as date_created, company_id
FROM calls
GROUP BY company_id
) t ON t.company_id = co.id
JOIN calls ca ON ca.company_id = t.company_id AND t.date_created = ca.created_at
WHERE co.customer_id = ?
EDIT:
the issue is you have more than one call per company at the max date. to test this just pull out one customer and company and look at the results.
SELECT co.id, co.label, ca.result, ca.id, ca.created_at as most_recent_date
FROM companies co
LEFT JOIN
( SELECT MAX(created_at) as date_created, company_id
FROM calls
GROUP BY company_id
) t ON t.company_id = co.id
JOIN calls ca ON ca.company_id = t.company_id AND t.date_created = ca.created_at
WHERE co.customer_id = ? AND co.id = ?
run this query and specify a specific company. look at the move_recent_date column and see if the date is the same for each row and if it is the max date

Answer 2

You can do this by joining on calls twice, the first time being in a subquery where you retrieve the last call date for each company:
SELECT * FROM companies co
LEFT JOIN (SELECT company_id, MAX(created_at) AS last_call FROM calls GROUP BY company_id) AS last_calls ON last_calls.company_id = co.id
LEFT JOIN calls ca ON ca.company_id = last_calls.company_id AND ca.created_at = last_calls.last_call
WHERE co.customer_id = ?
GROUP BY co.id

Answer 3

It looks like I found the answer.
This one gives the correct result, and still fast enough (0.27 seconds)
SELECT co.*, v.*
FROM companies co
LEFT JOIN
(
SELECT
ca.*
FROM calls ca
JOIN
(
SELECT
company_id,
MAX(created_at) AS max_created_at
FROM calls
GROUP BY company_id
) t
ON ca.company_id = t.company_id AND ca.created_at = t.max_created_at
GROUP BY company_id
) v ON co.id = v.company_id
Thanks everybody!

Answer 4

You should make a subselect to get max created from calls and use that as join condition to company and calls tables.
SELECT
co.id AS company_id,
co.name AS company_name,
ca.id AS lastcall_id,
ca.result AS lastcall_result
FROM companies AS co
LEFT JOIN calls AS ca
ON co.id = ca.company_id
INNER JOIN
(
SELECT
company_id,
MAX(created_at) AS max_created_at
FROM calls
GROUP BY company_id
) AS max_created_per_company
ON ca.company_id = max_created_per_company.company_id
AND ca.created_at = max_created_per_company.created_at
WHERE co.customer_id = ?

Answer 5

You could double join to the lastcall table, like in this example:
select companies.id, companies.name , lastcall.id, lastcall.result from companies
inner join (select max(created_at) as lastcall, company_id from calls group by company_id) maxcalls
on (companies.id = maxcalls.company_id)
inner join lastcall on (lastcall = created_at and companies.id = lastcall.company_id)
where customer_id = ?
select companies.* , ca.* from companies
inner join (select max(created_at) as lastcall, company_id from calls group by company_id) maxcalls on (companies.id = maxcalls.company_id)
inner join calls ca on (maxcalls.lastcall = ca.created_at and companies.id = ca.company_id)
where customer_id = ?