Laravel Sum transactions amount grouped by category - laravel

Question

I'm trying to sum all transactions and group these records by category with the summed value of transactions for a particular month.
Example (Current month):
Groceries => 500
Subscriptions => 100
Coffee => 0
But the current code I have only groups categories when a transaction has occurred for that month and will return the following:
Groceries => 500
Subscriptions => 100
Here are my the following database tables:
Transactions table:
id
date
category_id
amount
1
2021-01-01
1
200
2
2021-01-01
1
300
3
2021-01-01
2
100
3
2020-06-06
3
100
Categories table:
id
display_name
description
1
Groceries
Food
2
Subscriptions
Subscriptions
3
Coffee
Coffee
And my code that selects transactions for the current month but doesn't return a full list of categories even if there are no transactions.
$currentMonth = date('m');
$grouped = DB::table('categories')
->join('transactions', function ($join) use ($currentMonth) {
$join->on('transactions.category_id', '=', 'categories.id')
->whereRaw('MONTH(date) = ?',[$currentMonth]);
})
->groupBy('categories.display_name')
->selectRaw('sum(transactions.amount) as sum, display_name as name')
->pluck('sum', 'name');

Answer 1

Move the criteria in the WHERE clause to the ON clause of the join, and use a left join:
$grouped = DB::table('categories c')
->leftJoin('transactions t', function ($join) {
$join->on('t.category_id', '=', 'c.id')
$join->on(DB::raw('MONTH(t.date) = ?'))
})
->groupBy('c.display_name')
->selectRaw('SUM(t.amount) AS sum, c.display_name AS name')
->setBindings([$currentMonth])
->pluck('sum', 'name');
The raw MySQL query I am suggesting here is:
SELECT
c.display_name,
SUM(t.amount) AS sum
FROM categories c
LEFT JOIN transactions t
ON c.id = t.category_id AND
MONTH(t.date) = ?
GROUP BY
c.display_name;
The structure used above in the left join is critical here, because it ensures that every category on the left side of the join would appear in the result set, even if it have no matching transactions for a given month.