How to select one value from a two table join? - php

Question

I'm trying to achieve something in Laravel/MySQL and cannot seem to be pointed in the right direction for a solution. I can achieve what I am looking for with subqueries, but I have been told they are not as efficient as joins. And, I'm going to have to convert the solution for this into Eloquent/Query Builder, and the way I have it working with subqueries and unions doesn't seem to convert easily.
What I am trying to do is select one row from two possible tables, based on the created_at date of the row. I want to join this created_at value with my users table as a new column called started_at. Here is some sample data and how I can achieve the query with a subquery/union of the two possible tables that I can get the data from:
CREATE TABLE users (
id INTEGER,
first_name TEXT,
last_name TEXT
);
INSERT INTO users (id, first_name, last_name)
VALUES
(1, 'Craig', 'Smith'),
(2, 'Bill', 'Nye'),
(3, 'Bloop', 'Blop');
CREATE TABLE old_activity (
id INTEGER,
user_id INTEGER,
firm_id INTEGER,
amount INTEGER,
created_at DATE
);
INSERT INTO old_activity (id, user_id, firm_id, amount, created_at)
VALUES
(1, 1, 3, 5.24, '2019-04-29'),
(2, 2, 7, 4, '2019-03-28'),
(3, 3, 4, 6.99, '2019-04-28');
CREATE TABLE new_activity (
id INTEGER,
user_id INTEGER,
firm_id INTEGER,
plays INTEGER,
saves INTEGER,
created_at DATE
);
INSERT INTO new_activity (id, user_id, firm_id, plays, saves, created_at)
VALUES
(1, 1, 3, 10, 1, '2019-04-27'),
(2, 2, 3, 12, 2, '2019-03-29'),
(3, 3, 3, 6, 3, '2019-04-27');
CREATE TABLE firms (
id INTEGER,
name TEXT
);
INSERT INTO firms (id, name)
VALUES
(1, 'apple'),
(2, 'banana'),
(3, 'orange');
select
id,
first_name,
last_name,
(select created_at from old_activity
where user_id = users.id
union
select created_at from new_activity
where user_id = users.id
order by created_at asc
limit 1) as started_at
from users
The query should only return the oldest created_at for a particular user in one of the two activity tables.
How can I achieve this with a join? Any help with this would be greatly appreciated.

Answer 1

Hmmm . . . You could always use window functions:
select u.*, a.*
from users u left join
(select a.*,
row_number() over (partition by a.user_id order by a.created_at desc) as seqnum
from ((select oa.* from old_activity oa) union all
(select na.* from new_activity na)
) a
) a
on a.user_id = a.id and a.seqnum = 1