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Eloquent Performance: Subqueries

Published 14 September, 2020

In the previous post, I asked a the following question:

How can you get the latest login date from App\Models\User in an efficient way?

The short answer is by using the subqueries.

Subquery is a query within another query, and it's supported by most RDBMS such as MySQL, Postgres etc.

Let's demystify this by an example:

SELECT
    `users`.*,
    (
        SELECT
            COUNT(id)
        FROM
            `posts`
        WHERE
            `user_id` = `users`.`id`
        LIMIT 1) AS `total_posts`
FROM
    `users`
ORDER BY
    `total_posts` DESC
LIMIT 15 OFFSET 0

The above query will count the total number of posts for each user, so we don't need to have another SELECT COUNT()... statement.

Laravel supports subqueries through the addSelect method:

$users = User::query()
    ->addSelect([
        'total_posts' => Post::selectRaw('COUNT(*)')
            ->whereColumn('user_id', '=', 'users.id')
            ->take(1)
    ])
    ->orderBy('total_posts', 'desc')
    ->paginate();

Remember to join the tables correctly when using subqueries.

We've got a new column total_posts in the App\Models\User, that's amazing, isn't it?

Let me show you one more thing before I end this section.

It's better to move your subqueries into scopes, so you will have great control over them:

// App\Models\Post

public function scopeWithTotalPosts()
{
    $users = User::query()
        ->addSelect([
            'total_posts' => Post::selectRaw('COUNT(*)')
                ->whereColumn('user_id', '=', 'users.id')
                ->take(1)
        ])
        ->orderBy('total_posts', 'desc')
        ->paginate();
}

You can easily call the scope as follows:

User::withTotalPosts()->paginate();

Getting the latest login date using subquery

Now you know how to use subqueries in Laravel, this means that you can apply the same thing to get the latest login date.

Let's see how it's done:

// App\Models\User

public function scopeWithLatestLoginDate($query)
{
    $query->addSelect(['latest_login_date' => Login::select('created_at')
            ->whereColumn('user_id', 'users.id')
            ->latest()
            ->take(1)
        ])
        ->withCasts(['last_login_at' => 'datetime']);
}
// App\Http\Controllers\UsersController

$users = User::query()
    ->withLatestLoginDate()
    ->paginate(10);

Now, you can access the latest_login_date directly from the App\Models\User:

@foreach ($users as $user) {
    {{ $user->last_login_at->diffForHumans() }}
}

The latest_login_date is an instance of Illuminate\Support\Carbon and that's because it was casted using the withCasts method.

In the next upcoming post, I will discuss the fake relationships using subqueries.

Edit on Github

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