How to have a union in subquery MySQL Laravel? - php

Question

I'm trying to perform a subquery in Laravel to get some relevant user data, but the data may be in one of two tables. I have a table for test_activity and live_activity which both have a created_at column. My goal is to get the oldest record from a combination of the data from the two tables.
Sample data:
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 test_activity (
id INTEGER,
user_id INTEGER,
created_at DATE
);
INSERT INTO test_activity (id, user_id, created_at)
VALUES (1, 1, '2019-04-29'), (2, 2, '2019-03-28'), (3, 3, '2019-04-28');
CREATE TABLE live_activity (
id INTEGER,
user_id INTEGER,
created_at DATE
);
INSERT INTO live_activity (id, user_id, created_at)
VALUES (1, 1, '2019-04-27'), (2, 2, '2019-03-29'), (3, 3, '2019-04-27');
Here is how I am trying the query with Laravel:
$firstActivity = TestActivity::select('created_at')
->whereColumn('user_id', 'id');
$firstActivity = LiveActivity::select('created_at')
->whereColumn('user_id', 'id')
->union($firstActivity)
->limit(1)
->getQuery();
$users = Users::select(['id', 'first_name', 'last_name'])
->whereIn('id', $arrayOfIds)
->selectSub($firstActivity, 'start_date')
->paginate(25);
Here is the query as it's being executed and throwing an error:
select
`id`,
`first_name`,
`last_name`,
((select `created_at`
from `live_activity`
where `user_id` = `id`)
union
(select `created_at`
from `test_activity`
where `user_id` = `id`
limit 1) as `start_date`
from `users`
where `id` in (....)
limit 25
offset 0)
The error I get is this
Query 1 ERROR: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'union
How can I make this work? And, is there a better, more efficient way to perform this kind of query with Laravel? Thank you.
UPDATE:
I've provided sample data for anyone willing to help. Here is the query I am now trying to run:
select
id,
first_name,
last_name,
(select created_at
from test_activity
where user_id = users.id
union
select created_at
from live_activity
where user_id = users.id
order by created_at asc
limit 1) as start_date
from users;
How can I convert this to eloquent?

Answer 1

In a lot of cases, joins are faster than sub-queries. I recommend working with sql, and testing what is faster before writing it in Laravel, since you aren't using the ORM anyway, and then just rewrite the best query using the query building language.
The join approach would join all three columns, using join where MAX(created_at) on both the testactivity and liveactivity and then using select GREATEST(test_Created_at, live_create_at) as start_date