Feature: Support Union

[kassah]

This request is to add a feature for SQL Unions.

Laravel Docs: Laravel Eloquent Unions
MySQL Docs: MySQL 8.1 UNION Clause

Union Example

Example SQL input:

(select * from `users` where `last_name` is null) union (select * from `users` where `first_name` is null)

Example Eloquent output:

DB::table('users')
->whereNull('last_name')
->union(
            DB::table('users')
            ->whereNull('first_name')
)
->get();

Union All Example

Example SQL input:

(select * from `users` where `last_name` is null) union all (select * from `users` where `first_name` is null)

Example Eloquent output:

DB::table('users')
->whereNull('last_name')
->unionAll(
            DB::table('users')
            ->whereNull('first_name')
)
->get();

Union with orderBy on id

Example SQL input:

(select * from `users` where `last_name` is null) 
union
(select * from `users` where `first_name` is null) 
order by `id` asc

Example Eloquent output:

DB::table('users')
->whereNull('last_name')
->unionAll(
            DB::table('users')
            ->whereNull('first_name')
)
->orderBy('id')
->get();

Additional Context

While Eloquent doesn't handle returning a UNION from multiple models well (everything will be shoved into the first model's class), it does handle it well as a subselect with multiple models. It makes it much easier to scan across many different models as part of a very complex where.