Joining 3 tables and compute average of a column - php

Question

I am currently learning Laravel and I am using DB class when joining the three tables. I am able to join the three tables but I need to get the average of a certain column for a teacher (ratings table, rating column), here's what I have and I am stuck here.
Here's my table design
And here's my query
$teachers = DB::table('ratings as r')
->join('users as u','r.teacher_id', '=', 'u.id')
->join('user_infos as ui','r.teacher_id', '=', 'ui.user_id')
->select('r.rating','ui.about','ui.first_name','ui.last_name','ui.avatar','ui.rate','u.*')
->where('u.status', 1)->get();
Also, the results for same user is being repeated. The user has two ratings from the ratings table and it appear two times in my view.
What I want to display here is the list of all teachers and in each card, with their corresponding ratings.. so if I have two teachers in the table, it will display the two teachers and on the right top side of the card is their rating.

Answer 1

Here is a possible solution:
$teachers = DB::table('ratings as r')
->join('users as u','r.teacher_id', '=', 'u.id')
->join('user_infos as ui','r.teacher_id', '=', 'ui.user_id')
->select(DB::raw('AVG(r.rating) as average_rating'),'ui.about','ui.first_name','ui.last_name','ui.avatar','ui.rate','u.*')
->groupBy('r.teacher_id')
->where('u.status', 1)->get();

Answer 2

Ok..since you are using Laravel naming convention/recommendation, I think would be easier/cleaner if you use Eloquent.
I'm not sure if you already created the Eloquent models. And, because of that, I'll put everything here (models, etc).
Ratings model
class Rating extends Model
{
protected $guard = ['id'];
public function teacher()
{
return $this->belongsTo(User::class, 'teacher_id');
}
public function student()
{
return $this->belongsTo(User::class, 'student_id');
}
}
User info model
class UserInfo extends Model
{
protected $guard = ['id'];
public function user()
{
return $this->belongsTo(User::class);
}
}
User model
class User extends Model
{
protected $guard = ['id'];
public function ratings()
{
return $this->hasMany(Rating::class, 'teacher_id');
}
public function infos()
{
return $this->hasMany(UserInfo::class);
}
}
Possible query solution for your problem:
$ratings = Rating::with(['teacher.infos', 'student.infos'])->whereHas('teacher', function($q) {
$q->where('status', true);
})->get();
This will probably give you something like this:
// ratings: [
// "id": 1,
// "teacher_id": 1,
// "student_id": 2,
// ....
// "teacher": {
// "id": 1,
// "name": "...."
// ...
// "infos": [{
// "id": 1,
// "skype": '....'
// ....
// }]
// },
// "student": {
// "id": 2,
// "name": ....,
// "infos": [{
// "id": ...
// }]
// }
// ]
Now you have a collection of Ratings. And, if you need to access the user or the user info, you just need to
// Example:
$firstStudentInfo = $ratings->isEmpty() ? null : $ratings->first()->student->infos;
If you need to calculate something, you can either use an extra query (db) or just a single method on your
collection. I think, in this case, a collection can be faster. You can also create a specific collection
for your "Ratings" model, with specific calculations ("RatingsCollection").
Another example (blade template). Since we already loaded everything "eager load", we don't need to worry
with N+1 query problems here. (https://laravel.com/docs/5.5/eloquent-relationships#eager-loading)
// Ratings grouped by teacher
#foreach ($ratings->groupBy('teacher') as $rating)
Teacher: {{ $rating->avg()...}} or whatever..
#endforeach
If you still want to use DB, #Md Mahfuzur Rahman will do the trick.
:)